feat(speckit): integrate github/spec-kit for spec-driven development (#38)

Installs spec-kit slash commands (/speckit-specify, /speckit-plan,
/speckit-tasks, /speckit-implement, /speckit-analyze, /speckit-clarify,
/speckit-checklist, /speckit-constitution, /speckit-taskstoissues) into
Claude Code via .claude/skills/speckit-*/.

Note: specify-cli v1.0.0 init fails on Windows because spec-kit v0.5+
releases no longer attach binary zip assets to GitHub releases (confirmed
by inspecting all 30 releases via API; only v0.4.4 and earlier have
assets). Templates were sourced directly from the github/spec-kit@v0.7.3
repo tree and SKILL.md files were built per the claude integration spec
in src/specify_cli/integrations/claude/__init__.py (user-invocable:true,
disable-model-invocation:false, argument-hint injection).

Enables the "specs in git" workflow for cross-device work state:
- New feature → /speckit-specify → specs/NNN/spec.md
- Design → /speckit-plan → specs/NNN/plan.md
- Work → /speckit-tasks → specs/NNN/tasks.md (handoff artifact)
- Implement → /speckit-implement → code

Preserves existing CLAUDE.md (@AGENTS.md bridges intact) and existing
memory policies in AGENTS.md. memory/constitution.md is annotated at
the top to reference AGENTS.md for project-specific constraints.

No change to server.mjs, models.json, package.json, or any other code.

Co-authored-by: Tao Deng <dtzp555@gmail.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.specify/templates/checklist-template.md

@@ -0,0 +1,40 @@
+# [CHECKLIST TYPE] Checklist: [FEATURE NAME]
+
+**Purpose**: [Brief description of what this checklist covers]
+**Created**: [DATE]
+**Feature**: [Link to spec.md or relevant documentation]
+
+**Note**: This checklist is generated by the `/speckit.checklist` command based on feature context and requirements.
+
+<!-- 
+  ============================================================================
+  IMPORTANT: The checklist items below are SAMPLE ITEMS for illustration only.
+  
+  The /speckit.checklist command MUST replace these with actual items based on:
+  - User's specific checklist request
+  - Feature requirements from spec.md
+  - Technical context from plan.md
+  - Implementation details from tasks.md
+  
+  DO NOT keep these sample items in the generated checklist file.
+  ============================================================================
+-->
+
+## [Category 1]
+
+- [ ] CHK001 First checklist item with clear action
+- [ ] CHK002 Second checklist item
+- [ ] CHK003 Third checklist item
+
+## [Category 2]
+
+- [ ] CHK004 Another category item
+- [ ] CHK005 Item with specific criteria
+- [ ] CHK006 Final item in this category
+
+## Notes
+
+- Check items off as completed: `[x]`
+- Add comments or findings inline
+- Link to relevant resources or documentation
+- Items are numbered sequentially for easy reference