feat: add Pi coding agent support (-a pi)

[Shanvit7]

Summary

Adds Pi (pi / omegon binary) as a supported agent backend for expect.

What changed

packages/agent

• detect-agents.ts — registers pi with binaries ["pi", "omegon"] and skill dir .pi/skills
• acp-client.ts — adds AcpAdapter.layerPi: reads ~/.pi/agent/auth.json to check auth, resolves pi-acp adapter package at runtime, and adds a per-adapter inactivityTimeoutMs field (pi's extended-thinking mode can silently reason for several minutes before emitting its first token — its timeout is raised to 10 min vs the 3 min default for other agents)
• agent.ts — wires Agent.layerPi and adds "pi" to the AgentBackend union

packages/shared

• models.ts — adds "pi" to AgentProvider literals and display names
• launched-from.ts — adds PI_CODING_AGENT_DIR env var for auto-detection

Tests

• detect-agents.test.ts — detects pi via pi binary, detects via omegon fallback
• acp-adapter.test.ts — layerPi resolves adapter, unauthenticated/not-installed error messages

Runtime dependency

Pi's ACP adapter is the published pi-acp npm package (^0.0.24), loaded at runtime via require.resolve (same pattern as other ACP adapters). No bundling changes needed.

Usage

expect -a pi -m "smoke test the homepage" -u http://localhost:3000
# or let it read the git diff automatically:
expect -a pi -u http://localhost:3000

Tested

Ran against a live Next.js project with real git changes. Pi generated a 15-step test plan from the diff, executed all steps in ~4 minutes, and reported 8 passed / 7 failed with a session replay URL.

[Shanvit7]

Github base branch sideeffect. Keepin main fork clean