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/init.test.cjs

@@ -981,6 +981,40 @@ describe('cmdInitPhaseOp fallback', () => {
     assert.strictEqual(output.has_plans, false);
   });
 
+  test('fallback resolves drifted project-code-prefixed roadmap heading by bare number (#1455)', () => {
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'ROADMAP.md'),
+      '# Roadmap\n\n### Phase MANIFOLD-117: Prefixed Heading\n**Goal:** Build prefixed phase\n**Plans:** TBD\n'
+    );
+
+    const result = runGsdTools('init phase-op 117', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.phase_found, true);
+    assert.strictEqual(output.phase_dir, null);
+    assert.strictEqual(output.phase_number, '117');
+    assert.strictEqual(output.phase_name, 'Prefixed Heading');
+    assert.strictEqual(output.phase_slug, 'prefixed-heading');
+  });
+
+  test('fallback resolves drifted project-code-prefixed roadmap heading by prefixed ID (#1455)', () => {
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'ROADMAP.md'),
+      '# Roadmap\n\n### Phase MANIFOLD-117: Prefixed Heading\n**Goal:** Build prefixed phase\n**Plans:** TBD\n'
+    );
+
+    const result = runGsdTools('init phase-op MANIFOLD-117', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.phase_found, true);
+    assert.strictEqual(output.phase_dir, null);
+    assert.strictEqual(output.phase_number, 'MANIFOLD-117');
+    assert.strictEqual(output.phase_name, 'Prefixed Heading');
+    assert.strictEqual(output.phase_slug, 'prefixed-heading');
+  });
+
   test('prefers current milestone roadmap entry over archived phase with same number', () => {
     const archiveDir = path.join(
       tmpDir,