fix(#1455): resolve project-code-prefixed roadmap headings

* fix: remove hardcoded phase project-code prefix cap

Centralize project-code prefix stripping/matching and replace fixed {1,6} caps so long codes (for example MANIFOLD-117) resolve across phase, roadmap parser, roadmap upgrade, and validate flows.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: address review feedback on prefix parsing

- allow project_code prefixes with digits and underscores while preserving milestone parsing
- use shared optional project-code prefix source in phase dir parsing
- extend regression coverage for APP1/APP_1 prefixed phases

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* chore: resolve review nit in phase-id test comment

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: resolve project-code-prefixed roadmap headings

Make getRoadmapPhaseInternal recover from drifted project-code-prefixed ROADMAP headings while preserving canonical bare-heading preference. Add init.phase-op and parser regressions for #1455 and guard gsd-roadmapper against emitting project_code in headings.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* chore: add changeset for project-code phase fix

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* chore: update roadmapper agent size baseline

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix(#1455): tighten project-code prefix regex to [A-Z] start; add boundary test; document source order

Resolves blockers from review:
- Regex changed from [A-Z_][A-Z0-9_]* to [A-Z][A-Z0-9_]* so leading
  underscores (_FOO-7, _-7) are never misread as project-code prefixes;
  adds boundary test asserting both do NOT strip.
- Adds comment to roadmapPhaseLookupSources explaining why 3 sources are
  needed (order-dependent canonical-heading preference).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Solvely-Colin <211764741+Solvely-Colin@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/roadmap-parser.test.cjs

@@ -250,6 +250,52 @@ describe('roadmap-parser: getRoadmapPhaseInternal', () => {
     assert.strictEqual(result.goal, 'Set up infrastructure');
   });
 
+  test('finds drifted project-code-prefixed headings by bare number (#1455)', () => {
+    writeRoadmap(tmpDir, [
+      '## v1.0: Current',
+      '### Phase MANIFOLD-117: Prefixed Heading',
+      '**Goal:** Recover from roadmapper heading drift',
+    ].join('\n'));
+
+    const result = getRoadmapPhaseInternal(tmpDir, '117');
+    assert.ok(result !== null, 'bare number lookup should tolerate a prefixed heading');
+    assert.strictEqual(result.found, true);
+    assert.strictEqual(result.phase_number, '117');
+    assert.strictEqual(result.phase_name, 'Prefixed Heading');
+    assert.strictEqual(result.goal, 'Recover from roadmapper heading drift');
+  });
+
+  test('finds drifted project-code-prefixed headings by prefixed query (#1455)', () => {
+    writeRoadmap(tmpDir, [
+      '## v1.0: Current',
+      '### Phase MANIFOLD-117: Prefixed Heading',
+      '**Goal:** Exact prefixed lookup works on init resolver',
+    ].join('\n'));
+
+    const result = getRoadmapPhaseInternal(tmpDir, 'MANIFOLD-117');
+    assert.ok(result !== null, 'prefixed lookup should resolve the matching prefixed heading');
+    assert.strictEqual(result.found, true);
+    assert.strictEqual(result.phase_number, 'MANIFOLD-117');
+    assert.strictEqual(result.phase_name, 'Prefixed Heading');
+    assert.strictEqual(result.goal, 'Exact prefixed lookup works on init resolver');
+  });
+
+  test('prefers canonical bare heading before prefixed drift fallback (#1455)', () => {
+    writeRoadmap(tmpDir, [
+      '## v1.0: Current',
+      '### Phase MANIFOLD-117: Prefixed Heading',
+      '**Goal:** Drift fallback',
+      '',
+      '### Phase 117: Bare Heading',
+      '**Goal:** Canonical bare',
+    ].join('\n'));
+
+    const result = getRoadmapPhaseInternal(tmpDir, '117');
+    assert.ok(result !== null, 'bare lookup should resolve');
+    assert.strictEqual(result.phase_name, 'Bare Heading');
+    assert.strictEqual(result.goal, 'Canonical bare');
+  });
+
   test('returns null for missing phase number', () => {
     writeRoadmap(tmpDir, '### Phase 1: Foo\n**Goal:** bar\n');
     const result = getRoadmapPhaseInternal(tmpDir, '99');