enhance(#1960): add RCA branching (fishbone + AND-gate) to gsd-debugger (#2405)

* test(#1960): add failing-first RCA-branching contract + schema-invariant tests

Epic #1957 Phase 2A. Source-text-is-the-product contract tests (fishbone
>=2 categories, AND-gate, multi-cause root_cause, backward compat, reasoning
checkpoint candidate_causes+and_gate fields, debugger-philosophy single-cause
note, DEBUG template) plus behavioral schema-invariant checks on two fixtures:
two contributing causes (AND-gate yes) -> both recorded; single-cause
(AND-gate no) -> one root_cause, identical to today.

Failing-first: reference, agent edits, and template note do not yet exist.

* feat(#1960): add RCA branching (fishbone + AND-gate) to gsd-debugger

Epic #1957 Phase 2A. Guards against 5-Whys single-cause bias: before committing
root_cause, the debugger enumerates candidate causes across >=2 Ishikawa
categories (code/config/environment/data) and explicitly answers an AND-gate
question. When the AND-gate fires, every contributing cause is recorded, so a
multi-cause fix no longer recurs via the unaddressed second cause.
Resolution.root_cause may hold one OR a small set (additive; single-cause
sessions are byte-identical to today). The Structured Reasoning Checkpoint gains
candidate_causes + and_gate fields; debugger-philosophy.md adds the
single-cause-bias trap.

Full rules extracted to gsd-core/references/debugger-rca-branching.md (slim
Phase 2 routing + 2 checkpoint fields kept in the agent). INVENTORY + manifest +
agent-size baseline + install-parity goldens + AGENTS.md + DEBUG template updated.

* fix(#1960): address orthogonal review (AND-gate self-consistency, parity guard, narrowed claim, ripples)

- Reference: the collapse rule now enforces AND-gate self-consistency —
  and_gate=yes with a single confirmed cause is flagged as incomplete
  (return to Phase 3); a race/timing note clarifies such bugs bridge
  categories; the 'byte-identical' backward-compat claim narrowed to
  'root_cause shape unchanged; reasoning_checkpoint gains 2 fields in every
  session'.
- DEBUG.md: stale 'five-field' mirror prose -> seven-field (parallel-surface
  drift the reviewer flagged); new debug-session-management parity test pins
  the field-count claim to the gsd-debugger.md YAML keys (CRLF-safe).
- Scalar-assuming consumers of set-valued root_cause updated: session-manager
  compact summaries (319/332), diagnose-only return (1062), archive entry
  (1216), ROOT CAUSE FOUND return (1322).
- Test: added the AND-gate-yes/single-cause invariant + fixture; rephrased the
  fixture describe block honestly as a schema-invariant specification.
- Phase 2 bullet phrasing clarified ('at hypothesis formation, before the
  Phase 4 commit').

* test(#1960): parity regex accepts word-form count ('seven-field' or '7-field')

* test(#1960): parity regex counts array-valued YAML keys (no inline value)

* chore(#1960): backfill changeset pr number (PR #2405)

docs/AGENTS.md

@@ -402,6 +402,7 @@ runs its default whole-repo scan.
 - Requires human verification before marking resolved
 - Runs a multi-signal fix-acceptance guardrail (mutation check, no-op/deletion detector, adjacent tests, revert-and-reconfirm) before accepting a fix; degrades gracefully when Stryker or a test suite is absent
 - Ranks suspect code by Ochiai suspiciousness from test pass/fail coverage (spectrum-based fault localization) before forming hypotheses; skips cleanly when no coverage exists
+- Branches root-cause analysis across ≥2 Ishikawa categories and applies an AND-gate check before committing root_cause (guards against 5-Whys single-cause bias); root_cause may hold a set when the AND-gate fires
 - Appends to persistent knowledge base on resolution
 - Consults knowledge base on new sessions
 

docs/INVENTORY-MANIFEST.json

@@ -216,6 +216,7 @@
       "continuation-format.md",
       "debugger-fix-acceptance.md",
       "debugger-philosophy.md",
+      "debugger-rca-branching.md",
       "debugger-sbfl.md",
       "decimal-phase-calculation.md",
       "doc-conflict-engine.md",

docs/INVENTORY.md

@@ -296,6 +296,7 @@ Full roster at `gsd-core/references/*.md`. References are shared knowledge docum
 | `debugger-philosophy.md` | Evergreen debugging disciplines loaded by `gsd-debugger`. |
 | `debugger-fix-acceptance.md` | Multi-signal fix-acceptance guardrail (anti-overfitting) loaded by `gsd-debugger`. |
 | `debugger-sbfl.md` | Spectrum-based fault localization (Ochiai) pre-filter loaded by `gsd-debugger`. |
+| `debugger-rca-branching.md` | RCA branching (fishbone + AND-gate) anti-single-cause discipline loaded by `gsd-debugger`. |
 | `mandatory-initial-read.md` | Shared required-reading boilerplate injected into agent prompts. |
 | `agent-skills-bootstrap.md` | Shared agent_skills self-load contract (query + Read + dedup guard) injected into all 22 consumer agents. |
 | `project-skills-discovery.md` | Shared project-skills-discovery boilerplate injected into agent prompts. |