fix(#2043): reject single-digit slug word in phase-token extraction (all sites)

A phase whose slug's first word is a bare single digit (dir
`46-6-rs-pipeline-orchestrator`, roadmap phase "6 Rs Pipeline Orchestrator" →
slug `6-rs-…`) had its phase token over-collected as `46-6` instead of `46`, so
`gsd-tools` phase-by-number lookups (init.plan-phase, init.phase-op, and
downstream execute/verify/ship) resolved phase_dir=null / has_context=false.

Root cause: an over-broad "looks numeric" test (`/^\d/`, `\d+(?:-\d+)*`)
classified token segments and could not distinguish a legitimate zero-padded
sub-phase segment (`01`, `02`) from a single-digit slug word (`6`). Zero-padded
phase/sub-phase segments are always ≥2 digits, so requiring ≥2 digits is the
structural distinguisher. Applied consistently across every same-class
implementation the triage identified (fixing extractPhaseToken alone leaves the
health-check and milestone-filter subsystems exposed):

- src/phase-id.cts extractPhaseToken — a pure-numeric leading segment continues
  only with ≥2-digit segments; a letter-prefixed milestone id (`M1`) still
  admits its single-digit sub-phase (`M1-2`).
- src/validate.cts PHASE_TOKEN_FROM_DIR_RE (W005/W006/W007 health checks) and
  canonicalPlanStem (I001 plan/summary pairing).
- src/roadmap-parser.cts isDirInMilestone numericRe (getMilestonePhaseFilter —
  highest blast radius: a false negative silently excludes a phase from the
  milestone).
- src/core-utils.cts + its verbatim duplicate src/phase.cts extractCanonicalPlanId.

Regression tests added across tests/{phase-id,health-validation,core-utils,
roadmap-parser}.test.cjs using the shared `46-6-rs-…` fixture, asserting the
token resolves to `46` (not `46-6`) while legit multi-segment tokens
(`01-02`, `02-03-04`, `M1-2`, `68-01`, `02-01`) are unchanged. The
roadmap-parser test exercises the real getMilestonePhaseFilter end-to-end.

The residual ≥2-digit-leading-slug case (e.g. `NN-2024-roadmap`) stays out of
scope — subsumed by the #612 / #565 phase-ID convention work, per the issue.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/core-utils.test.cjs

@@ -533,6 +533,21 @@ describe('extractCanonicalPlanId', () => {
     assert.ok(typeof result === 'string');
     assert.ok(result.length > 0);
   });
+
+  test('rejects single-digit slug word (#2043)', () => {
+    // "46-6-rs-pipeline-orchestrator" is not a valid phase-token filename shape
+    // (the "6" is a slug word, not a sub-phase segment) — must not collapse to
+    // the pre-fix, buggy "46-6".
+    assert.notStrictEqual(
+      coreUtils.extractCanonicalPlanId('46-6-rs-pipeline-orchestrator'),
+      '46-6',
+    );
+    // Legit multi-segment phase-token filenames are still extracted correctly,
+    // including a single-digit letter-suffix phase id ("3A") — the "3A" token is
+    // found and paired with its zero-padded plan index, not left unpaired.
+    assert.strictEqual(coreUtils.extractCanonicalPlanId('01-02-PLAN.md'), '01-02');
+    assert.strictEqual(coreUtils.extractCanonicalPlanId('3A-01-feature-PLAN.md'), '3A-01');
+  });
 });
 
 // ─── countMatchedSummaries (#1988) ───────────────────────────────────────────