feat: posthog plugin service

[skoob13]

Need to ship the skills first before merging the PR.

TLDR:

This PR ships the embedded plugin for Claude Code and skills sync to Codex (as Codex doesn't have a plugin system yet). Skills are automatically pulled from posthog/posthog. The next step might be to reuse the same service for Wizard.

Also a few important changes:

• The MCP is pinned to version 2–it ships the new PostHog data discovery through a unified HogQL interface.
• Sets Claude Code's feature flag to offload all MCP tools to a virtual CLI, enabling progressive discovery. It works very well with skills.

Problem

Twig agents (Claude Code, Codex) need access to PostHog-specific skills, but currently there's no mechanism to deliver or update these skills. We need a plugin system that:

1. Ships a baseline set of skills with the app
2. Keeps skills up-to-date without requiring app releases
3. Supports local skill development for contributors
4. Works across both Claude Code (via plugins) and Codex (via ~/.agents/skills/)

Changes

New PostHog plugin system (plugins/posthog/) — a directory-based plugin format with plugin.json, MCP/LSP configs, and skill directories. Skills are markdown files (SKILL.md) that get passed to agents as context.

Three-tier skill sourcing with priority layering:

Source	Location	Priority
Shipped	plugins/posthog/skills/ (committed)	Lowest
Remote	Downloaded from GitHub releases (skills.zip)	Middle
Local dev	plugins/posthog/local-skills/ (gitignored)	Highest (dev only)

PosthogPluginService (apps/twig/src/main/services/posthog-plugin/service.ts) — injectable singleton that manages the full plugin lifecycle at runtime:

• On startup: copies bundled plugin to userData, overlays any cached remote skills, syncs to Codex directory, starts a 30-minute update timer
• On update: downloads skills.zip, extracts with atomic swap (.new/.old dirs to avoid partial states), re-assembles the runtime plugin, re-syncs to Codex
• Includes throttling, reentrance guards, and graceful error handling (download failures are non-fatal — existing skills are preserved)

Build-time Vite plugin (copyPosthogPlugin in vite.main.config.mts) — at bundle time, copies the plugin directory to the build output, downloads and overlays remote skills, and in dev mode overlays local-skills/ on top. Watches all three sources for hot-reload.

Agent integration (AgentService) — injects PosthogPluginService and passes getPluginPath() to Claude Code sessions via the plugins option. Also adds x-posthog-mcp-version: 2 header to the PostHog MCP server config, and sets ENABLE_EXPERIMENTAL_MCP_CLI env var.

Dev tooling — pnpm skills:pull script (scripts/pull-skills.mjs) downloads remote skills into local-skills/ for local editing and iteration.

Packaging — updated Forge config to unpack plugins/posthog/** from ASAR so the plugin directory is accessible at runtime.

Tests

Manual testing & service unit tests