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/SKILL.md

@@ -51,13 +51,11 @@ Both clauses are non-negotiable.
 
 ## Dispatch
 
-Dispatch `debugger` with:
-
-| Carrier field | Content |
-|---------------|---------|
-| `symptom` | Exact error message, stack trace, wrong output, or repro command |
-| `repro_known` | One-line repro if known, otherwise the agent finds one |
-| `recent_iter` | Iteration that last touched the suspected area (often `git log` tail) |
+Dispatch the `debugger` agent with the carrier fields it defines
+under **Carrier contract** in `agents/debugger.md` — `symptom`,
+`repro_known`, `recent_iter`. That table is the authoritative
+definition of those fields; it is deliberately not restated here,
+so the two files cannot drift.
 
 The agent writes the RED test to the working tree (uncommitted)
 and reports the handoff carrier for `implement` mini-mode. The
@@ -75,13 +73,11 @@ RED+GREEN at the end of mini-mode).
 
 ## Handoff Contract
 
-`debug` produces, for `implement` mini-mode:
-
-| Field | Content |
-|-------|---------|
-| `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"` |
+`debug` produces, for `implement` mini-mode, the handoff carrier
+the agent defines under **Output format** in `agents/debugger.md`
+— `red_test_path`, `cause_summary`, `constraint`. That list is
+the authoritative definition of those fields; it is deliberately
+not restated here, so the two files cannot drift.
 
 Anything else (broader refactor, doc rewrite, new feature) is
 OUT of scope for the bug-fix iteration and gets queued for a
@@ -91,8 +87,11 @@ separate one.
 
 - **Agent dispatched:** `agents/debugger.md` — carries the
   four-phase process, the Phase 4.5 escalation rule, the
-  Common Rationalisations table, and the Red Flags list. The
-  orchestrator does not execute these phases directly.
+  Common Rationalisations table, and the Red Flags list. It is
+  also the single source for the carrier and handoff field
+  definitions (Carrier contract / Output format); this skill
+  names the fields but does not redefine them. The orchestrator
+  does not execute these phases directly.
 - **Hand-off target:** `../implement/SKILL.md` — runs the
   GREEN side after the orchestrator has decided whether to
   commit the RED test separately or as part of the final fix

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.