fix(#1384): scan live changeset fragments in the #1777 purity gate (#1385)

The product-name-purity gate scanned CHANGELOG.md only, never the
.changeset/*.md fragments that render into it. An impure fragment passed
PR review, sat dormant, and re-introduced a forbidden parenthetical
product description at the next release — even after CHANGELOG.md had been
hand-fixed. This is the recurrence vector behind the 1.5.0 back-merge (#1379)
failure.

- Purify the two live fragments to the already-accepted forms:
  - happy-finches-travel.md: "Claude Code (background dispatch …)"
    -> "Claude Code; background dispatch …"
  - 924-claude-flat-skill-layout.md: "Claude (`~/.claude/…`)"
    -> "Claude at `~/.claude/…`"
- Extend the #1777 gate to also scan live .changeset/*.md fragments,
  reusing one shared detection helper. Archived fragments never re-render
  and are intentionally out of scope.

Test-only + changeset-prose change; no production behavior change.

Closes #1384

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

.changeset/924-claude-flat-skill-layout.md

@@ -2,4 +2,4 @@
 type: Fixed
 pr: 924
 ---
-**Claude global install reverted to flat skill layout so concrete skills are discoverable.** PR #883 introduced nested skill layout for Claude (`~/.claude/skills/gsd-ns-<router>/skills/<stem>/SKILL.md`), but Claude Code's skill discovery scans only one level under `~/.claude/skills/` — nested concrete skills were never listed in the Skill-tool available-skills list and direct `Skill(skill="gsd-plan-phase")` calls stopped working. This fix reverts Claude to the flat layout (`~/.claude/skills/gsd-<name>/SKILL.md`) so all ~61 concrete skills are top-level and immediately discoverable. The 6 other runtimes that confirmed non-recursive scanning (cline, qwen, hermes, augment, trae, antigravity) retain their nested layout. (#924)
+**Claude global install reverted to flat skill layout so concrete skills are discoverable.** PR #883 introduced nested skill layout for Claude at `~/.claude/skills/gsd-ns-<router>/skills/<stem>/SKILL.md`, but Claude Code's skill discovery scans only one level under `~/.claude/skills/` — nested concrete skills were never listed in the Skill-tool available-skills list and direct `Skill(skill="gsd-plan-phase")` calls stopped working. This fix reverts Claude to the flat layout (`~/.claude/skills/gsd-<name>/SKILL.md`) so all ~61 concrete skills are top-level and immediately discoverable. The 6 other runtimes that confirmed non-recursive scanning (cline, qwen, hermes, augment, trae, antigravity) retain their nested layout. (#924)

.changeset/happy-finches-travel.md

@@ -2,4 +2,4 @@
 type: Fixed
 pr: 863
 ---
-**`/gsd-manager` and `/gsd-autonomous --interactive` no longer silently skip worktree isolation and independent verification on Claude Code.** They dispatched plan/execute as background agents, but a backgrounded Claude Code agent has no Agent/Task tool and cannot spawn the nested executors, plan-checker, or verifier — so isolation and verification silently never ran. Both workflows now resolve the runtime and run plan/execute inline on Claude Code (background dispatch is kept on runtimes that support nested subagents).
+**`/gsd-manager` and `/gsd-autonomous --interactive` no longer silently skip worktree isolation and independent verification on Claude Code.** They dispatched plan/execute as background agents, but a backgrounded Claude Code agent has no Agent/Task tool and cannot spawn the nested executors, plan-checker, or verifier — so isolation and verification silently never ran. Both workflows now resolve the runtime and run plan/execute inline on Claude Code; background dispatch is kept on runtimes that support nested subagents.