refactor: single-source the debug carrier and handoff contracts

The carrier (symptom/repro_known/recent_iter) and handoff
(red_test_path/cause_summary/constraint) field definitions were
tabled in full in both SKILL.md and debugger.md — the exact
duplication that let the `constraint` wording drift earlier.

Since the debugger agent runs in a fresh context, it must carry
both contracts inline regardless; that makes debugger.md the
natural single source. Mark its Carrier-contract and Output-format
tables authoritative, fold in the richer field descriptions that
only SKILL.md had, and reduce SKILL.md to naming the fields plus a
pointer — no field semantics restated. Drift-prone prose now lives
in exactly one place. Consistent with SKILL.md's own stated split
(debugger.md = source of truth; skill = trigger/dispatch/handoff).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

debug/agents/debugger.md

@@ -41,6 +41,9 @@ the dispatching skill file does not duplicate it.
 
 ## Carrier contract — what the controller hands you
 
+This table is the authoritative definition of the carrier fields;
+the dispatching skill references it rather than restating it.
+
 | Field | Content |
 |-------|---------|
 | `symptom` | Exact error message, stack trace, wrong output, or repro command |
@@ -203,9 +206,12 @@ At most 250 words, structured:
 - **Cause:** file + function + why (1-2 sentences).
 - **RED test:** path to the new test in the working tree
   (uncommitted).
-- **Handoff carrier for `implement` mini-mode:**
-  - `red_test_path`: absolute path
-  - `cause_summary`: 1-2 sentences
+- **Handoff carrier for `implement` mini-mode** (this list is
+  the authoritative definition of the handoff fields; the
+  dispatching skill references it rather than restating it):
+  - `red_test_path`: absolute path to the failing test file —
+    minimal and autonomous (Phase 3)
+  - `cause_summary`: 1-2 sentences naming the file + function + why
   - `constraint`: `"minimal fix, no surrounding cleanup, no opportunistic refactor"`
 - **Concerns / blockers:** if applicable.