docs(edit-workflow): fix set_file_slice rule + add contract-change check

Three surgical fixes to conductor/edit_workflow.md:

1. **§2 "Verify Before Editing"** — removed the leftover
   `git checkout -- src/gui_2.py` instruction. The user's
   commit `4eba059e unfuck edit workflow` removed most of
   the git checkout nuke instructions but missed §2. The
   revised §2 now says: read the contract (function signature,
   yield shape, return type) before editing, and DO NOT use
   `git checkout` to revert. Ask the user.

2. **§3 "Reading Before Editing"** — added the line-number
   offset check. `set_file_slice` uses 1-indexed inclusive
   `start_line`/`end_line`; off-by-one is a common silent
   failure. The rule is now: confirm the exact line range
   with `get_file_slice` first.

3. **§8 "set_file_slice IS Valid for Multi-Line Content
   (Revised 2026-06-09)"** — replaced the wrong rule
   ("Do not use set_file_slice for multi-line content") with
   the correct rule: set_file_slice IS valid for 3-10 line
   surgical edits, with a tool-selection guide (which tool
   for which job), a mandatory contract-change check
   (search for callers of the symbol being changed; update
   all callers in the same atomic commit if the public
   interface changes), and a mandatory whitespace-and-EOL
   rule (preserve line ending, indentation, and line count).

4. **§9 "No Diagnostic Noise in Production Code
   (Added 2026-06-09)"** — new section. Diag stderr goes
   to log files or /tmp scripts, NOT src/*.py. If you must
   add diag lines to production code, they are part of the
   same atomic commit as the fix — they do not live
   uncommitted in the working tree.

5. **"If set_file_slice produces wrong indentation"** —
   new handler in the Step-by-Step Workflow. Tells the
   agent: you wrote the wrong indent; the tool did what
   you asked; re-read the file with get_file_slice; do
   NOT use git checkout to revert.

These are the rule corrections the user demanded after
the Tier-2's bad set_file_slice + git nuke + diag-noise
behavior. Markdown only. No code modified.

conductor/edit_workflow.md

@@ -16,16 +16,19 @@ The `manual-slop_edit_file` tool requires **exact string matches** (character-fo
 Before ANY edit to a function you haven't touched recently:
 
 ```
-1. Run: git checkout -- src/gui_2.py
-2. Run: py_check_syntax on src/gui_2.py
-3. Get current state with get_file_slice
+1. Run: py_check_syntax on src/<file>.py
+2. Get current state with get_file_slice (the exact lines you're about to touch)
+3. Read the contract: does this function/field/method's signature, yield shape, or return type have callers I need to update?
 ```
 
+DO NOT use `git checkout` or `git restore` to "revert" your way to a clean state. That destroys in-progress work. If a previous edit left the file in a broken state, ask the user.
+
 ### 3. Reading Before Editing (CRITICAL)
 
-- Use `get_file_slice` to get the EXACT text including all whitespace
+- Use `get_file_slice` to get the EXACT text including all whitespace and EOL
 - Copy text directly from the tool output - do NOT reformat
-- If using get_definition, verify the text matches before editing
+- If using `get_definition`, verify the text matches before editing
+- For `set_file_slice`: confirm the exact `start_line` and `end_line` (1-indexed, inclusive) by reading the file first. Off-by-one is a common silent failure.
 
 ### 4. The Edit Tool Parameters (snake_case)
 
@@ -72,9 +75,47 @@ This keeps the `@property` attached to its original method.
 2. Instantiate the class
 3. Call the new method in the way it's expected to be called (`ctrl.foo_ts` for a property, `ctrl.foo_ts()` for a method)
 
-### 8. Do Not Use `set_file_slice` For Multi-Line Content (Added 2026-06-07)
+### 8. `set_file_slice` IS Valid for Multi-Line Content (Revised 2026-06-09)
 
-`set_file_slice` does literal line replacement by design. It does not reindent, does not normalize EOL, does not parse decorators. Use it for surgical line-level edits (3-10 lines). If you need to insert or replace a multi-method block, use `manual-slop_edit_file` with verified exact-text old_string/new_string, or use `py_add_def` / `py_update_definition` for class/method-level work.
+The previous rule ("Do not use set_file_slice for multi-line content") was wrong. `set_file_slice` does literal line replacement by design and is the right tool for 3-10 line surgical edits.
+
+**When to use which tool:**
+
+- **`set_file_slice`** for surgical 3-10 line edits where you know the exact line range. Verify the line range with `get_file_slice` first. The `start_line` and `end_line` are 1-indexed and inclusive. The new content must reproduce the line count exactly (or be a precise replacement of the same N lines).
+- **`manual-slop_edit_file`** for exact-string replacement when you don't know the line range, or when the edit has a unique anchor string.
+- **`py_update_definition`** for whole-function replacement (AST-detected).
+- **`py_add_def`** for adding a new method/class to a class.
+- **`py_remove_def`** for removing a method/class.
+
+**The contract-change check (mandatory for any edit that changes a public interface):**
+
+Before any edit, search the codebase for callers of the function/symbol/yield shape you're changing. If your edit changes:
+- A function signature (add/remove/rename a parameter)
+- A return type or yield shape (e.g. `yield process, gui_script` → `yield process, gui_script, workspace_path`)
+- A class hierarchy (add/remove a base class, change a method's name)
+- A module-level function name (rename)
+- A public attribute name
+
+...you MUST update ALL callers in the same atomic commit. Use `py_find_usages` to locate them. If you change a contract and don't update callers, you have broken the codebase.
+
+**The whitespace-and-EOL rule (mandatory for set_file_slice):**
+
+The `new_content` must preserve:
+- The file's line ending convention (CRLF on Windows, LF on Linux — pick from the surrounding file, not from your text editor's default)
+- The indentation of the surrounding code (1 space per level, per `conductor/code_styleguides/python.md` §1)
+- The number of lines replaced (`start_line`..`end_line` must equal `len(new_content.splitlines())`)
+
+If you mismatch any of these, the file will fail to parse. Run `py_check_syntax` and a real `import` after every `set_file_slice`.
+
+### 9. No Diagnostic Noise in Production Code (Added 2026-06-09)
+
+`sys.stderr.write(f"[XYZ_DIAG] ...")` lines added to `src/*.py` for debugging are technical debt the moment they ship. If you need to instrument for a one-time investigation:
+
+- Write the diag output to a log file: `tests/artifacts/<test_name>.diag.log`
+- Or to a standalone diagnostic script under `/tmp/diag_<name>.py` that imports the production module and exercises it
+- Or read the production source with `get_file_slice` and reason about it directly
+
+Do NOT add diag lines to `src/*.py` "temporarily." If you must add them for a single test run, they are part of the same atomic commit as the fix — they do not live uncommitted in the working tree. If you "revert everything," that means the diag lines are also reverted.
 
 ## Step-by-Step Workflow for gui_2.py
 
@@ -96,9 +137,16 @@ get_file_slice path=src/gui_2.py start_line=X end_line=Y
 
 - The text you're trying to replace doesn't EXACTLY match
 - Use `get_file_slice` to get the exact text
-- Copy it character-for-character including whitespace
+- Copy it character-for-character including whitespace and EOL
 - Try again with exact match
 
+### If `set_file_slice` produces wrong indentation:
+
+- You wrote the wrong indent in `new_content`. The tool did what you asked.
+- Re-read the file with `get_file_slice` to confirm the surrounding indent
+- Rewrite the `new_content` with the correct indent
+- Do NOT use `git checkout` to "revert"
+
 ## Alternative: Update Definition Approach
 
 For large function rewrites, use `py_update_definition`: