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)

tests/debug-session-management.test.cjs

@@ -187,8 +187,37 @@ describe('debug skill dispatch and sub-orchestrator (#2148, #2151)', () => {
   });
 
   test('debug.md delegates to gsd-debug-session-manager', () => {
-    const content = fs.readFileSync(path.join(process.cwd(), 'gsd-core/workflows/debug.md'), 'utf8');
+    const content = fs.readFileSync(path.join(process.cwd(), 'gsd-core', 'workflows', 'debug.md'), 'utf8');
     assert.ok(content.includes('gsd-debug-session-manager'),
       'debug.md does not delegate to session manager');
   });
+
+  test('DEBUG.md reasoning_checkpoint field-count claim matches gsd-debugger.md YAML keys (parity)', () => {
+    // Parallel-surface drift guard (DEFECT.GENERATIVE-FIX): the field-count
+    // claim in the DEBUG.md section_rules prose must equal the number of keys
+    // enumerated in the gsd-debugger.md reasoning_checkpoint YAML block.
+    // CRLF-safe regex/split per DEFECT.WINDOWS-TEST-PORTABILITY. Accepts either
+    // digit ("7-field") or word ("seven-field") count form.
+    const NUMWORDS = { one: 1, two: 2, three: 3, four: 4, five: 5, six: 6, seven: 7, eight: 8, nine: 9, ten: 10 };
+    const debugContent = fs.readFileSync(path.join(process.cwd(), 'gsd-core', 'templates', 'DEBUG.md'), 'utf8');
+    const agentContent = fs.readFileSync(path.join(process.cwd(), 'agents', 'gsd-debugger.md'), 'utf8');
+    const debugMatch = debugContent.match(/reasoning_checkpoint[^.\r\n]*?(\d+|one|two|three|four|five|six|seven|eight|nine|ten)-field structured reasoning record/i);
+    assert.ok(debugMatch, 'DEBUG.md must state a "N-field structured reasoning record" claim for reasoning_checkpoint');
+    const claimedCount = /^\d+$/.test(debugMatch[1]) ? parseInt(debugMatch[1], 10) : NUMWORDS[debugMatch[1].toLowerCase()];
+    assert.ok(typeof claimedCount === 'number', `unrecognized field-count token: ${debugMatch[1]}`);
+    const yamlBlock = agentContent.match(/reasoning_checkpoint:\s*\r?\n([\s\S]*?)```/);
+    assert.ok(yamlBlock, 'gsd-debugger.md must define a fenced reasoning_checkpoint YAML block');
+    const keys = new Set();
+    for (const line of yamlBlock[1].split(/\r?\n/)) {
+      // Match 2-space-indented YAML keys (with OR without an inline value —
+      // array-valued keys like confirming_evidence: have no trailing space).
+      const m = line.match(/^ {2}([a-z_]+):/);
+      if (m) keys.add(m[1]);
+    }
+    assert.strictEqual(
+      keys.size,
+      claimedCount,
+      `parity drift: DEBUG.md claims ${claimedCount}-field but gsd-debugger.md enumerates ${keys.size} keys (${[...keys].join(', ')}). Update BOTH surfaces together.`
+    );
+  });
 });