fix(#730): scope current-milestone Phase Details section in roadmap parser (#748)

`extractCurrentMilestone()` scoped the current-milestone window to its
`## Phases` checklist subsection and terminated at the milestone's own
`## Milestone … (Phase Details)` heading, so the `### Phase N:` detail
headers fell outside scope. Every parser-backed command — `init.phase-op`
(and thus `/gsd:discuss-phase`, `/gsd:plan-phase`), `state`, `roadmap list`,
and `validate health` (W006) — therefore could not resolve phases of any
milestone after the first until a `.planning/phases/` directory already
existed, blocking discuss/plan.

The parser now additionally includes the current milestone's `(Phase Details)`
section in scope, located via the already-computed version matches and anchored
(boundary-aware) to the selected milestone's version token so sibling
sub-milestones sharing a version prefix do not cross-pollinate. The existing
heading selection and primary window are unchanged.

Adds tests/bug-730-milestone-phase-details-scope.test.cjs covering the
two-milestone reproduction, first-milestone non-regression, direct
getRoadmapPhaseInternal resolution, validate-health W006 visibility, a
three-milestone roadmap, and the closed-sibling sub-milestone case.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/lint-test-file-count.allowlist.json

@@ -43,6 +43,7 @@
     },
     "milestone": {
       "files": [
+        "bug-730-milestone-phase-details-scope.test.cjs",
         "milestone-archive.test.cjs",
         "milestone-helper.test.cjs",
         "milestone-prefixed-convention.test.cjs",