feat(setup): add Pi extension integration via engram setup pi

[yvolchkov]

📋 Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

🔍 Problem Description

Pi (pi-coding-agent (https://github.com/mariozechner/pi-coding-agent)) is a growing coding agent that supports native Type Script extensions. Currently there's no way to integrate Engram with Pi — users would need to manually set up MCP, which adds friction and doesn't leverage Pi's extension system (lifecycle hooks, prompt injection, native tools).

💡 Proposed Solution

Add first-class Pi support via engram setup pi, following the same pattern as the existing OpenCode integration (native TypeScript extension, no MCP required).

  engram setup pi

This installs ~/.pi/agent/extensions/engram.ts, which provides:

• Native Engram tools — engram_search, engram_save, engram_context, engram_session_summary, engram_timeline, engram_get, engram_update, engram_delete, engram_stats
• Memory Protocol injection into the system prompt (same protocol as other agents)
• Session lifecycle hooks — session start/end tracking via /sessions
• Compaction recovery — deferred summary persistence + context reinjection after session_compact
• Auto-start of engram serve when needed
• Privacy stripping — tags removed before any data leaves the extension

📦 Affected Area

CLI (commands, flags)

🔄 Alternatives Considered

• MCP-only integration: Pi supports MCP, but using a native extension avoids MCP setup friction and enables prompt injection + lifecycle hooks that MCP can't provide. This mirrors the rationale behind the OpenCode plugin.
• Separate repository: Could ship the extension standalone, but embedding it in the engram binary (like OpenCode) keeps the install to a single command.

📎 Additional Context

The implementation follows the established OpenCode plugin pattern exactly:

• Source of truth in plugin/pi/engram.ts
• Embedded copy generated via go generate ./internal/setup/
• Installed to user home (~/.pi/agent/extensions/) by engram setup pi
• All non-fatal error handling, privacy stripping, and session management patterns match the existing plugins