refactor(#2128): single-source 23 phase-token re-derivations; sanction 14 context-specific sites

Route 23 literal re-derivations of the canonical phase-number token through
phase-id.cjs `PHASE_NUMBER_TOKEN_SOURCE` (via new RegExp). Each conversion was
proven BYTE-IDENTICAL (old.source === new.source && old.flags === new.flags), so
the runtime regexes are unchanged — zero behavior change by construction.

The remaining 14 phase-token sites are genuine but context-specific and stay
literal with a `// phase-id-owner: <reason>` sanction: dir-name parses whose
dash-continuation semantics differ from extractPhaseToken, and the [A-Za-z]
case-variant / [.-] dot-or-dash separator forms that are not source-byte-equal
to the canonical token.

Scanner (`npm run check:phase-id-drift`) is now green.

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

src/milestone.cts

@@ -23,7 +23,7 @@ import ioMod = require('./io.cjs');
 const { output, error } = ioMod;
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import phaseIdMod = require('./phase-id.cjs');
-const { escapeRegex, normalizePhaseName, phaseTokenMatches } = phaseIdMod;
+const { escapeRegex, normalizePhaseName, phaseTokenMatches, PHASE_NUMBER_TOKEN_SOURCE } = phaseIdMod;
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import roadmapParserMod = require('./roadmap-parser.cjs');
 const { getMilestonePhaseFilter, extractCurrentMilestone, getMilestoneInfo } = roadmapParserMod;
@@ -177,7 +177,7 @@ function cmdMilestoneComplete(cwd: string, version: string, options: MilestoneCo
         const roadmapContent = fs.readFileSync(roadmapPath, 'utf-8');
         const scopedContent = extractCurrentMilestone(roadmapContent, cwd);
         // #1729: `(?:\s*\([^)\n]*\))?` tolerates a pre-colon ( ) tag (literal mirror of OPTIONAL_PHASE_TAG_SOURCE).
-        const phasePattern = /#{2,4}\s*Phase\s+(\d+[A-Z]?(?:\.\d+)*)(?:\s*\([^)\n]*\))?\s*:\s*([^\n]+)/gi;
+        const phasePattern = new RegExp(`#{2,4}\\s*Phase\\s+(${PHASE_NUMBER_TOKEN_SOURCE})(?:\\s*\\([^)\\n]*\\))?\\s*:\\s*([^\\n]+)`, 'gi');
         const noDirectoryPhases: string[] = [];
         let pm: RegExpExecArray | null;
         const phaseDirEntries = ((): string[] => {