feat(directives): harvest 2 directives from error_handling.md (Result pattern + nil-sentinel)

conductor/directives/nil_sentinel_pattern/v1.md

@@ -0,0 +1,33 @@
+# nil_sentinel_pattern — v1
+
+**Why this iteration:** Lifted verbatim from `conductor/code_styleguides/error_handling.md` §"The 5 Patterns" #1 (lines 23-47).
+This is the baseline encoding — the convention-style description with code example currently in production.
+Future variants will test alternative encodings (rationale-first, before/after, tabular) against this baseline.
+
+**Source:** `conductor/code_styleguides/error_handling.md:23-47`
+
+---
+
+### 1. Nil-Sentinel Dataclasses (replaces `None`)
+
+When a function would "return None" in conventional Python, return a
+nil-sentinel dataclass instead. The sentinel has all default values
+(zero-initialized) and is safe to read from.
+
+```python
+from dataclasses import dataclass, field
+
+@dataclass(frozen=True)
+class NilPath:
+ exists: bool = False
+ read_text: str = ""
+ errors: list[ErrorInfo] = field(default_factory=list)
+
+NIL_PATH = NilPath() # module-level singleton
+```
+
+Callers don't need `if x is None:` checks; they can call `x.read_text` and
+get `""` on the nil path.
+
+**Convention:** `NIL_*` (uppercase) is the module-level singleton. `Nil*`
+(PascalCase) is the class. Frozen dataclass prevents runtime mutation.