fix(#2135): anchor milestone heading regex + strip delimiter, widen preserve guard

getMilestoneInfo's `##` heading regex was unanchored (no `^`/`m`), so it
matched a `##` quoted mid-line inside a Milestones bullet and captured a
delimiter-led fragment into milestone_name — clobbering the curated name on
every phase transition.

roadmap-parser.cts (load-bearing):
- Consult the 🚧 name-bearing marker FIRST (reorder; it already existed but was
  shadowed by a spuriously-successful heading match).
- Anchor the `##` regex to line start (`^` + `m` flag) so a heading quoted
  in backticks/prose can no longer match.
- stripLeadingDelimiter removes a leading em/en-dash/colon/hyphen run that
  .trim() cannot (the `## vX.Y — Name` convention).

state.cts (defense in depth):
- Widen the #948 preserve guard from 'derived equals placeholder' to
  'derived does not look like a name' (non-empty, not placeholder, not
  punctuation-led), so a future bad derive preserves the curated name instead
  of silently overwriting it.

Closes #2135

src/state.cts

@@ -1573,8 +1573,18 @@ function syncStateFrontmatter(content: string, cwd: string | undefined): string
   // existing frontmatter already holds; only an empty derived value falls through
   // to this guard (the primary #905 preserve path below handles that).
   const MILESTONE_NAME_PLACEHOLDER = 'milestone';
+  // #2135: widen the preserve guard. A bad derive is not always the literal
+  // placeholder — getMilestoneInfo can return a delimiter-led fragment
+  // ("— Active Milestone") when the roadmap regex mis-binds. Preserve the
+  // existing curated name unless the derived value actually looks like a name:
+  // non-empty, not the placeholder, and not punctuation-led.
+  const derivedName = derivedFm['milestone_name'];
+  const derivedLooksLikeName = typeof derivedName === 'string'
+    && derivedName.length > 0
+    && derivedName !== MILESTONE_NAME_PLACEHOLDER
+    && !/^[\s—–:-]/.test(derivedName);
   if (
-    derivedFm['milestone_name'] === MILESTONE_NAME_PLACEHOLDER &&
+    !derivedLooksLikeName &&
     existingFm['milestone_name'] &&
     existingFm['milestone_name'] !== MILESTONE_NAME_PLACEHOLDER
   ) {