feat: add pi agent support

[ZeR020]

Summary

Add first-class support for the pi coding agent as codemem's fourth client integration, alongside opencode, Claude Code, and Codex.

Motivation

Pi is a deliberately minimal, TypeScript-native coding agent whose only plugin surface is its extension API (pi.on() events + pi.registerTool()); it has no native MCP support and no hooks mechanism — extensions loaded from npm/git/local "pi-packages" are the entire integration surface. Its extension API is arguably the richest of the four clients: per-turn before_agent_start handlers can append to the system prompt, tool_result middleware can patch results, and session_before_compact exposes compaction boundaries.

A pi integration built on codemem's existing thin-client pattern (HTTP to the viewer server + CLI fallback) lands at opencode-level first-classness, plus two pi-only capabilities:

• compaction-boundary memory extraction — flush extraction before pi discards context (observe-only; never replaces pi's own summarizer)
• fork/resume-aware raw-event streams — pi session ids mint per fork

Proposed scope

• @codemem/pi-extension (new npm pi-package, lockstep-versioned with @codemem/opencode-plugin): event ingest via POST /api/pi-hooks, per-turn memory injection via system-prompt append (never the persistent message channel), and codemem's 14 memory_* tools registered natively (no pi-mcp-adapter required; optional --pi-mcp opt-in for adapter users).
• core: pi event adapter (AdapterEvent v1 with source: "pi", deterministic event ids, flush-only compaction signals) + observer extraction configuration derived from pi's own settings.json/models.json/auth.json (API-key providers only in v1; OAuth reported as explicitly unsupported rather than silently 401).
• viewer-server: dedicated POST /api/pi-hooks (server-side source assertion) + closes the HTTP route gap so all 14 tools work over HTTP.
• cli: pi-hook-ingest (HTTP-first, spool + boundary flush) / pi-hook-inject commands, and codemem setup pi target — one-command install (--pi-only, --pi-mcp, --pi-extension-path).
• ui/docs: pi in settings/health client enumerations; README + docs updated in the same PR.

Because the store is shared, pi users who also run opencode/claude/codex get cross-agent memory automatically (pack filtering is project-scoped, never agent-scoped).

Non-goals (this iteration)

• OAuth-based extraction via pi providers (API-key providers only in v1).
• Google / non-OpenAI-or-Anthropic observer wire APIs.
• Replacing or customizing pi's compaction summarizer (observe-only).
• Generalizing /api/raw-events to arbitrary sources (dedicated route instead, mirroring /api/claude-hooks).

I have this implemented and reviewed (multi-pass adversarial review + full quality gates green) and will open the linked PR shortly.