feat: initial plugin — compaction proxy with lifecycle hooks

Transparent proxy that intercepts Claude Code's context compaction
and allows replacing it with custom logic (e.g. Morph's compaction API).

Architecture:
- src/proxy.ts: Bun HTTP server that detects compaction requests by
  matching the "create a detailed summary" marker in user messages.
  Normal requests pass through unchanged.
- src/transforms.ts: onCompact() handler — currently forwards to
  Anthropic as-is. Plug point for Morph integration.
- hooks/: SessionStart starts the proxy on a free port and sets
  ANTHROPIC_BASE_URL via CLAUDE_ENV_FILE. SessionEnd kills it.
- .claude-plugin/plugin.json: Plugin manifest for marketplace install.

Build: `bun build --compile src/proxy.ts --outfile bin/morph-proxy`
Dev:   `bun run dev`

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>