feat: debug RED stage requires minimal, autonomous fixture

Phase 3 previously said only "smallest possible reproducer" — a
passive property an agent can satisfy by name while still asserting
against the full reproducing fixture. Sharpen it into an explicit
two-step discipline: minimize the trigger (delta-reduce until one
more cut makes the symptom vanish) and make the fixture autonomous
(inline input, no shared fixtures/DB seeds/clocks/server boots
unless the bug is genuinely at that integration boundary).

- Iron Law gains: an un-minimized repro is not a RED test
- Phase 3 rewritten with explicit Minimize + Autonomous steps
- 3 rationalization rows + 4 red flags for the "full fixture
  reproduces deterministically, ship it" failure mode
- SKILL.md overview/handoff mirrored at orchestrator altitude

Baseline (writing-skills RED-first): 6 subagent runs showed the old
wording usually steered right but relied on agent instinct; GREEN:
2 runs under the targeted full-fixture pressure now reduce to the
minimal trigger and cite the contract verbatim.

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

debug/agents/debugger.md

@@ -54,6 +54,7 @@ If the symptom is vague ("something feels off"), return
 
 ```
 ROOT CAUSE FIRST. RED TEST SECOND. STOP.
+THE RED TEST IS MINIMAL AND AUTONOMOUS — AN UN-MINIMIZED REPRO IS NOT A RED TEST.
 NO FIX ATTEMPT IN THIS DISPATCH — THE GREEN SIDE GOES TO `implement` MINI-MODE.
 NO SYMPTOM SUPPRESSION. NO HUNCH-DRIVEN CHANGES.
 ```
@@ -103,18 +104,52 @@ Each phase completes before the next starts.
 
 ### Phase 3 — RED test
 
+A reproduction is not yet a RED test. The carrier's repro — or
+the full fixture a user handed you — is the *starting point*,
+not the artefact. Phase 3 isolates the specific failing case
+into a minimal, autonomous test. **A red test that reproduces
+but was never minimized does not satisfy this phase** — any old
+red test will not do.
+
 1. State your hypothesis as a falsifiable claim:
    *"The bug is X in `<path>:<fn>` because Y."*
-2. **Write the failing test FIRST**, before any fix attempt:
-   - smallest possible reproducer (placed in the testing
-     location idiomatic for this project — integration test,
-     unit test, E2E suite, etc.)
+2. **Minimize the trigger.** Reduce the reproducer to the
+   smallest input that still fails:
+   - Start from whatever reproduces — a 240-line fixture, a
+     booted server, a 400-cell workbook — and strip everything
+     not required to trigger the symptom. Delete keys, cells,
+     routes, lines; shrink values; collapse nesting — re-running
+     after each cut — until removing one more thing makes the
+     symptom vanish. What remains is the minimal trigger, and it
+     *is* the diagnosis: it names the bug more precisely than any
+     prose.
+   - A bug reproducing inside a large fixture is not licence to
+     assert against that fixture. Reduction is the work of this
+     phase, not optional polish to skip under time pressure.
+3. **Make the fixture AUTONOMOUS.** The test constructs its own
+   input inline and depends on nothing it does not itself set
+   up — no shared fixture files, DB seeds, frozen clocks, mock
+   services, or `beforeAll` server boots — *unless* the bug is
+   genuinely in that integration boundary. Phase 1's data-flow
+   trace decides which layer owns the bug, not which layer is
+   convenient: a pure-function bug gets a pure-function test
+   next to its unit, never an assertion bolted onto an E2E suite
+   that happens to exercise it.
+4. **Write the failing test FIRST**, before any fix attempt,
+   from the minimal autonomous trigger:
+   - placed in the testing location idiomatic for this project
+     (the layer the fix belongs to — usually a unit test; an
+     integration/E2E test only when Phase 1 located the bug at
+     that boundary)
    - automated, deterministic — same input always same output
    - doc comment names the *property* the test protects, not
      the symptom
-3. Run the test. Confirm it fails with the symptom the carrier
-   described.
-4. Leave the failing test in the working tree as an unstaged
+5. Run the test. Confirm it fails with the symptom the carrier
+   described — and that it fails at the diagnosed cause, not at
+   some unrelated earlier error. (A repro that no longer fires
+   once minimized means the trigger you blamed is wrong; return
+   to Phase 1.)
+6. Leave the failing test in the working tree as an unstaged
    change. You do NOT commit. The orchestrator decides commit
    shape; the suggested subject if the orchestrator elects to
    commit the RED separately is `test: red for <symptom>`.
@@ -196,6 +231,9 @@ At most 250 words, structured:
 | "End-of-day, just drop a fix into the working tree and write the test tomorrow" | End-of-day pressure is the worst time for shotgun fixes. Clean tree + open bug > three speculative half-fixes. Diagnosis is RED-first; the fix is a separate dispatch. |
 | "This bug is trivial, the existing tests will catch regressions" | The existing tests already passed while this bug shipped. By definition they don't cover it. |
 | "I can fix and verify in one go and be done" | The skill explicitly splits RED (you) and GREEN (`implement` mini-mode). Don't collapse them; the split is the whole point. |
+| "The full fixture reproduces 100% and the test is deterministic and automated — that's a valid RED test" | Deterministic ≠ minimal. A crash buried in a 240-line fixture pins the symptom to incidental data, breaks on unrelated fixture edits, and tells the implementer nothing about the trigger. Phase 3 reduction is mandatory, not polish. |
+| "Reducing the fixture is cleanup I can skip when I'm short on time" | The minimal trigger *is* the diagnosis — it names the bug. Skipping reduction means you don't yet know what the bug is, only that something somewhere fails. |
+| "The bug only shows with the real DB seed / booted server, so the test needs them too" | Almost always the integration is incidental to a unit-level defect. Phase 1's data-flow trace decides the layer, not convenience. If the failing unit is pure, the test is pure too. |
 
 ## Red Flags — STOP and return to Phase 1
 
@@ -204,6 +242,10 @@ At most 250 words, structured:
 - "I see the symptom, let me fix it"
 - "The test is redundant, I manually verified"
 - "Pattern says X but I'll adapt it differently"
+- "It reproduces, that's good enough" (without minimizing first)
+- RED test loads a large or shared fixture you did not reduce
+- Asserting against the full repro without isolating the minimal trigger
+- A bug in a pure function whose test needs `beforeAll` / a server boot / a DB seed
 - *"One more hypothesis"* (when ≥ 2 already failed)
 - About to apply ANY edit outside the new test file
 - About to run `git commit` (anywhere, ever — you never commit)