feat: add PostHog telemetry

[dimavedenyapin]

Summary

• Add a main-process PostHog telemetry service with an installation-scoped anonymous id, periodic batching, bounded buffering, and shutdown flush.
• Record boot, task lifecycle, agent session lifecycle, user turn, approval response, and stop-all telemetry with aggregate metadata only.
• Keep telemetry inert unless POSTHOG_KEY is configured; POSTHOG_HOST and telemetry buffering controls can be overridden through environment variables.

Local setup

• Export a PostHog project key before starting the Electron app: export POSTHOG_KEY=<project-key>.
• Optional overrides: export POSTHOG_HOST=https://us.i.posthog.com, export TELEMETRY_ENABLED=true, export TELEMETRY_FLUSH_BATCH_SIZE=20, export TELEMETRY_MAX_BUFFERED_EVENTS=1000.
• Start the app normally with pnpm dev; events flush from the main process to ${POSTHOG_HOST}/batch/.
• To disable telemetry locally, run with TELEMETRY_ENABLED=false or omit POSTHOG_KEY.

Release environment setup

• In GitHub, add POSTHOG_KEY as a repository or environment secret for the release environment.
• Add non-sensitive defaults as repository or environment variables if needed: POSTHOG_HOST, TELEMETRY_ENABLED, TELEMETRY_FLUSH_BATCH_SIZE, and TELEMETRY_MAX_BUFFERED_EVENTS.
• Wire the release workflow job to expose those values to the app build/release step, for example: env: { POSTHOG_KEY: ${{ secrets.POSTHOG_KEY }}, POSTHOG_HOST: ${{ vars.POSTHOG_HOST }}, TELEMETRY_ENABLED: ${{ vars.TELEMETRY_ENABLED }}}.
• GitHub Actions env vars are available to the release runner. If the packaged desktop app must include the key for end-user installs, the release pipeline must explicitly inject it into the packaged runtime environment or build-time config.

Test plan

• pnpm typecheck
• node node_modules/eslint/bin/eslint.js src/main/analytics-service.ts src/main/index.ts src/main/agent-manager.ts src/main/ipc-handlers.ts (passes with one existing warning in src/main/agent-manager.ts)

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