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>

src/phase-id.cts

@@ -205,9 +205,29 @@ function extractPhaseToken(dirName: string): string {
 
   const segments = rest.split('-');
   const tokenSegments: string[] = [];
+  // #2043: distinguish a real (zero-padded, ≥2-digit) phase/sub-phase segment
+  // from a single-digit slug word. A pure-numeric leading segment ("46") only
+  // continues with ≥2-digit segments, so "46-6-rs-…" yields "46" (the "6" is the
+  // slug's first word), not "46-6". Milestone-prefixed ids like "M1-2" reach here
+  // with "M1-" already stripped as a project-code prefix (see
+  // PROJECT_CODE_PREFIX_CAPTURE_RE_I), so "2" is the leading segment and the same
+  // pure-numeric rule applies (M1-46-6-rs → "M1-46"). The firstLetterPrefixed
+  // carve-out covers letter+digit leading segments that survive prefix stripping
+  // because of punctuation (e.g. "P0.3-2"), whose single-digit continuation is
+  // intentionally preserved (unchanged from prior behaviour).
+  let firstLetterPrefixed = false;
   for (let i = 0; i < segments.length; i++) {
     const seg = segments[i];
-    if (/^\d/.test(seg) || (i === 0 && /^[A-Za-z]{1,3}\d/.test(seg))) {
+    if (i === 0) {
+      if (/^\d/.test(seg)) {
+        tokenSegments.push(seg);
+      } else if (/^[A-Za-z]{1,3}\d/.test(seg)) {
+        tokenSegments.push(seg);
+        firstLetterPrefixed = true;
+      } else {
+        break;
+      }
+    } else if (/^\d{2,}/.test(seg) || (firstLetterPrefixed && /^\d/.test(seg))) {
       tokenSegments.push(seg);
     } else {
       break;