feat: Spec-Kit (github/spec-kit) integration support

[JARVIS-coding-Agent]

Summary

Add GitHub Spec-Kit integration to OpenAB, enabling spec-driven development workflows directly from Discord.

Motivation

Spec-Kit provides a structured approach to software development: write specs first, then let AI agents generate implementations. Integrating it into OpenAB allows teams to drive the full spec-driven workflow (constitution → specify → plan → tasks → implement) from Discord without switching tools.

Proposed Changes

1. Dockerfile — Pre-install spec-kit (all 4 images)

• Install uv + specify-cli@v0.6.0 in Dockerfile, Dockerfile.claude, Dockerfile.codex, Dockerfile.gemini
• Add ~/.local/bin to PATH

2. Discord command rewriting (direction 1)

• Intercept /speckit.* messages in discord.rs
• Rewrite into structured prompts the downstream agent can act on
• Supported commands: constitution, specify, plan, tasks, implement (with fallback for others)

3. Session context injection (direction 4)

• On session_new, read .specify/ directory in the agent working dir
• Inject constitution.md, spec.md, plan.md, tasks.md as context into the first prompt
• Silently skip if .specify/ does not exist

4. Configuration

[speckit]
rewrite_commands = true   # intercept /speckit.* commands
inject_context = true     # inject .specify/ on session init

Both default to true. Omitting the section keeps both enabled.

5. Documentation

• Add docs/speckit-integration.md covering usage, configuration, and setup

Files Changed

• Dockerfile, Dockerfile.claude, Dockerfile.codex, Dockerfile.gemini
• src/discord.rs
• src/acp/connection.rs
• src/acp/pool.rs
• src/config.rs
• src/main.rs
• config.toml.example
• docs/speckit-integration.md (new)

[JARVIS-coding-Agent]

Progress Update

PR #193 addresses the docs/skill portion of this issue:

• docs/skills/speckit-sdd.md — Agent-agnostic SDD workflow definition (Phase 0–5)
• docs/speckit-integration.md — Setup guide with per-agent install instructions (Kiro CLI, Claude Code, Codex, Gemini CLI)
• docs/skills/README.md — Directory index

What is covered by PR #193

• ✅ Skill-based SDD workflow (no code changes needed)
• ✅ Documentation for all 4 agents

What remains open (from the original proposal)

• ⬜ Pre-install specify-cli in Dockerfiles
• ⬜ Discord command rewriting (/speckit.* interception in discord.rs)
• ⬜ .specify/ context injection on session init (connection.rs / pool.rs)
• ⬜ [speckit] config section in config.toml

The skill-based approach was chosen as a first step because it requires zero code changes — users just copy the skill file to their agent's expected path. The remaining items can be addressed in follow-up PRs if needed.

[JARVIS-coding-Agent]

Design Decision: Skill-based approach over built-in integration

經過評估，決定 不將 Spec-Kit 整合為 OpenAB 的預設功能，改以 Skill 方式提供。原因如下：

OpenAB 的輕量化原則

OpenAB 的定位是 Discord ↔ Agent 的透明橋接器。在核心程式碼中加入特定開發流程的邏輯（command rewriting、context injection、config section）會增加耦合度，違背其輕量化設計。

SDD 的特殊性

Spec-Driven Development 是一種特定的開發方法論，並非所有使用者都需要。將它硬編碼進 OpenAB 意味著：

• 不使用 SDD 的使用者也要承擔額外的程式碼複雜度
• spec-kit 版本更新需要跟著改 OpenAB 程式碼
• 每新增一個開發流程就要改一次核心

Skill 方式的優勢

• 零程式碼修改 — 使用者按需複製 skill 檔案即可啟用
• Agent-agnostic — 同一份流程定義適用於 Kiro CLI、Claude Code、Codex、Gemini CLI
• 可獨立更新 — skill 檔案的更新不需要重新部署 OpenAB
• 使用者自主 — 不需要的人完全不受影響

因此，原 issue 中提到的 Dockerfile 預裝、discord.rs 攔截、config section 等程式碼層面的修改將不再進行。以 PR #193 的 docs/skill 方案作為此 issue 的最終解法。