feat(mcp): Add edit_file tool - native edit replacement that preserves indentation

- New edit_file(path, old_string, new_string, replace_all) function
- Reads/writes with newline='' to preserve CRLF and 1-space indentation
- Returns error if old_string not found or multiple matches without replace_all
- Added to MUTATING_TOOLS for HITL approval routing
- Added to TOOL_NAMES and dispatch function
- Added MCP_TOOL_SPECS entry for AI tool declaration
- Updated agent configs (tier2, tier3, general) with edit_file mapping

Note: tier1, tier4, explore agents don't need this (edit: deny - read-only)

.opencode/agents/tier2-tech-lead.md

@@ -34,6 +34,7 @@ You MUST use Manual Slop's MCP tools. Native OpenCode tools are unreliable.
 ### Edit MCP Tools (USE THESE)
 | Native Tool | MCP Tool |
 |-------------|----------|
+| `edit` | `manual-slop_edit_file` (find/replace, preserves indentation) |
 | `edit` | `manual-slop_py_update_definition` (replace function/class) |
 | `edit` | `manual-slop_set_file_slice` (replace line range) |
 | `edit` | `manual-slop_py_set_signature` (replace signature only) |