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-analyze/SKILL.md

@@ -4,6 +4,10 @@ description: Perform a non-destructive cross-artifact consistency and quality an
 argument-hint: "Optional focus areas for analysis"
 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/analyze.md
 ---
 
 ## User Input

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

@@ -4,6 +4,10 @@ description: Generate a custom checklist for the current feature based on user r
 argument-hint: "Domain or focus area for the checklist"
 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/checklist.md
 ---
 
 ## Checklist Purpose: "Unit Tests for English"

.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

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

@@ -4,6 +4,10 @@ description: Create or update the project constitution from interactive or provi
 argument-hint: "Principles or values for the project constitution"
 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/constitution.md
 ---
 
 ## User Input

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

@@ -4,6 +4,10 @@ description: Execute the implementation plan by processing and executing all tas
 argument-hint: "Optional implementation guidance or task filter"
 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/implement.md
 ---
 
 ## User Input

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

@@ -4,6 +4,10 @@ description: Execute the implementation planning workflow using the plan templat
 argument-hint: "Optional guidance for the planning phase"
 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/plan.md
 ---
 
 ## User Input

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

@@ -4,6 +4,10 @@ description: Create or update the feature specification from a natural language
 argument-hint: "Describe the feature you want to specify"
 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/specify.md
 ---
 
 ## User Input

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

@@ -4,6 +4,10 @@ description: Generate an actionable, dependency-ordered tasks.md for the feature
 argument-hint: "Optional task generation constraints"
 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/tasks.md
 ---
 
 ## User Input

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

@@ -4,6 +4,10 @@ description: Convert existing tasks into actionable, dependency-ordered GitHub i
 argument-hint: "Optional filter or label for GitHub issues"
 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/taskstoissues.md
 ---
 
 ## User Input