feat: support Pi Coding Agent as a presence source

[PerfectPan]

Summary

• Add @earendil-works/pi-coding-agent as a first-class presence source. A managed extension at ~/.pi/agent/extensions/agent-presence.ts bridges Pi's lifecycle events (before_agent_start, turn_start, tool_execution_*, agent_end, session_shutdown) into the existing CLI hook → state → render → provider pipeline.
• Active state gates on before_agent_start, not session_start, so an idle Pi TUI is never counted; Stop events go through spawnSync so quick pi -p runs don't leak state.
• setup installs the extension; uninstall removes only the managed file and only its own entry in settings.json#/extensions, leaving user-owned Pi extensions and other settings untouched.
• Render output gains pi N alongside codex / claude / gemini / opencode.

Files

• New: src/hooks/pi.ts, scripts/install-pi-extension.ts, scripts/uninstall-pi-extension.ts, test/pi-extension-install.test.ts, .changeset/pi-coding-agent-support.md.
• Updated: src/cli/hook-context.ts, src/installers.ts (extension source builder + idempotent install/uninstall coordination), src/setup.ts (script order), package.json (install/uninstall:pi-extension scripts; fills the missing install:gemini-hook in install:all-hooks), README.md, README.zh-CN.md, docs/architecture.md, skills/agent-presence/SKILL.md, and the existing hooks/installers/setup tests.

Test plan

• pnpm install --frozen-lockfile --ignore-scripts
• pnpm test — 15 test files / 114 tests passed (covers pi hook context, lifecycle transitions, render grouping with pi 1, extension source string contents, settings.json merge, tmp-HOME install/uninstall idempotency, refusal to overwrite a non-managed file)
• pnpm run typecheck
• pnpm run build
• pnpm pack --dry-run
• agent-presence setup --provider feishu-signature --skip-login --hook-command absolute — extension written with marker, user's other Pi extension preserved
• Real pi --provider zai --model glm-5.1 -p "Reply with exactly: pi-ok" returned pi-ok (exit 0)
• Mid-run local status showed "value": "1 个 AI 牛马正在搬砖 | pi 1" with active[0].source = "pi" against the real Pi session id; activeCount returned to 0 after Pi exited (no residual state)
• status --remote matched local; remote slot updated through the existing l.garyyang provider, no Pi-specific provider path
• Provider 429 rate-limit hits seen during sampling were absorbed by existing deferred-flush logic (result=rate-limited followed by a successful retry); Pi's own GLM call was unaffected
• Hygiene scan: only ZAI_API_KEY env-var name appears in docs; no real key, no glm.txt path, no byted/bnpm/private-token leaks; no dist/node_modules tracked

Follow-up risk

• Pi 0.75.5's -p mode can stall when stdin is wired to a long-lived pipe (observed when invoking pi from another agent CLI's sandbox). The fix in that case is pi … -p "…" </dev/null; normal terminal use is unaffected. The extension itself is unchanged either way — once before_agent_start fires the integration runs end-to-end.