feat: add octo (octo-agent) installer target

[Leihb]

Add support for octo (octo-agent) as a new CodeGraph installer target.

What changed

• New installer target: src/installer/targets/octo.ts
• Writes the CodeGraph MCP server config into octo's standard config file:
  • global: ~/.octo/mcp.json
  • local: ./.octo/mcp.json
• Also upserts the marker-fenced CodeGraph instructions block into octo's rules file:
  • global: ~/.octo/octorules.md
  • local: ./.octo/octorules.md
• Adds octo to the installer target registry and the TargetId union.
• Updates CLI help text, README, site docs, and CLAUDE.md to include octo in the supported-agent list.
• Updates CHANGELOG.md.

Why octorules.md

octo loads octorules.md into every session's system prompt, similar to how Claude Code reads CLAUDE.md. Adding the same short CodeGraph guidance block there helps octo remember to reach for codegraph_explore before grepping.

Verification

• npm run build ✅
• npx vitest run __tests__/installer-targets.test.ts ✅ (161 tests)
• Manually verified codegraph install --target=octo --location=global and codegraph uninstall --target=octo --location=global correctly generate and cleanly remove both mcp.json and the octorules.md section without touching existing user content.