docs(classification): move schema out of gitignored AGENTS/, into docs/

AGENTS/functions/CLAUDE.md is git-ignored local agent state, not part
of the repo -- a comment/commit referencing it as the schema's home
points contributors at a file they can't see. The canonical CLASSIFICATION
schema now lives at docs/function-classification-schema.md (tracked),
with CONTRIBUTING.md's existing function-header-conventions section
extended to introduce it, and the C1 shadow doc's pointer updated to
match. AGENTS/functions/CLAUDE.md keeps only a one-line pointer to the
tracked file instead of duplicating the definitions.

docs/manual/08-components-reference/01-c1-command-shadows.md

@@ -108,6 +108,7 @@ toggle state:
                                                              editor launch.
 
 A function's own doc header records which of these it depends on: see the
-`CLASSIFICATION` label (`uses-shadow(...)` / `bypasses-shadow(...)`) in
-`AGENTS/functions/CLAUDE.md`.
+`CLASSIFICATION` label (`uses-shadow(...)` / `bypasses-shadow(...)`),
+documented in full at
+[`docs/function-classification-schema.md`](../../function-classification-schema.md).