chore(speckit): add compatibility/metadata frontmatter to SKILL.md files (refs #39) (#50)

Backfill the two YAML frontmatter blocks that upstream
`specify_cli/agents.py:build_skill_frontmatter()` emits:

  compatibility: "Requires spec-kit project structure with .specify/ directory"
  metadata:
    author: github-spec-kit
    source: claude:templates/commands/<cmd>.md

Applied uniformly across all 9 speckit-* SKILL.md files. Each file gets the
two blocks appended to its existing frontmatter (between the closing fields
and the closing `---`). No functional change — these fields are informational
only, per #39 body and PR #38 reviewer note.

Verified:
- All 9 files have `compatibility:` key (grep -lc, 9/9)
- All 9 files have `metadata:` block with `author: github-spec-kit` (9/9)
- All 9 files have `source: claude:templates/commands/<cmd>.md` matching the
  command name (9/9)
- No existing fields modified or removed; pure additive +4 lines per file

Upstream reference (build_skill_frontmatter):
https://github.com/github/spec-kit/blob/main/src/specify_cli/agents.py

.claude/skills/speckit-clarify/SKILL.md

@@ -4,6 +4,10 @@ description: Identify underspecified areas in the current feature spec by asking
 argument-hint: "Optional areas to clarify in the spec"
 user-invocable: true
 disable-model-invocation: false
+compatibility: "Requires spec-kit project structure with .specify/ directory"
+metadata:
+  author: github-spec-kit
+  source: claude:templates/commands/clarify.md
 ---
 
 ## User Input