Multi-agent install: auto-detect 7 coding agents, upsert configs + hooks + instructions

- Detect Claude Code, Codex CLI, Gemini CLI, Zed, OpenCode, Antigravity, Aider
- Per-agent MCP config upsert: Codex TOML, OpenCode JSON, Antigravity JSON
- Instruction file upsert with marker-based sections (no duplicates on re-install)
- Claude Code PreToolUse hook (Grep|Glob|Read -> MCP reminder)
- Gemini CLI BeforeTool hook (grep_search|read_file -> MCP reminder)
- Fix Zed MCP config: args:[""] instead of broken source:"custom" (#24)
- Rewrite skill descriptions to directive pattern (100% auto-activation)
- Keyword-rich MCP tool descriptions for Tool Search discovery
- 35 new tests (agent detection, config upsert, instructions, hooks)
- All platforms green: macOS 2022, Linux 2004+18skip, Windows cross-compile OK

Author: DeusData