feat(core): add PostHog telemetry v1

[hachej]

Summary

• Implements PostHog telemetry v1 behind an explicit core-owned env helper.
• Adds tiny structural TelemetrySink contracts so agent/workspace remain PostHog-free.
• Wires env-only telemetry through the core-composed app/server path.
• Emits minimal safe server-side events for app open, failed requests, agent chat lifecycle, and tool completion/failure.
• Documents opt-in env vars, shared PostHog project prefixing, allowed properties, and deferred browser/runtime telemetry scope.

Privacy / AGENTS.md alignment

• Telemetry is off by default; POSTHOG_KEY alone is a no-op.
• Captured properties are default-deny allowlisted and sanitized centrally.
• No prompts, assistant output, file contents, command strings/output, raw paths, headers/cookies/tokens, env dumps, raw errors, or stack traces are captured.
• PostHog dependency/imports live only in @hachej/boring-core; agent/workspace use the structural sink only.
• No browser telemetry endpoint, runtime config, direct browser PostHog, Sentry/OTLP, or tenant/provider routing in v1.

Validation

• pnpm --filter @hachej/boring-core test src/server/telemetry/__tests__/posthog.test.ts src/app/server/__tests__/createCoreWorkspaceAgentServer.telemetry.test.ts src/app/server/__tests__/createCoreWorkspaceAgentServer.telemetry-smoke.test.ts
• pnpm --filter @hachej/boring-agent test src/shared/__tests__/telemetry.test.ts src/server/__tests__/createAgentApp.test.ts src/server/http/routes/__tests__/chat.test.ts src/server/harness/pi-coding-agent/__tests__/tool-adapter.telemetry.test.ts
• pnpm --filter @hachej/boring-workspace test src/shared/__tests__/telemetry.test.ts
• pnpm --filter @hachej/boring-agent typecheck
• pnpm --filter @hachej/boring-workspace typecheck
• pnpm --filter @hachej/boring-core typecheck
• pnpm lint:invariants

Review

• Fresh model review: SHIP (Claude), after fixing the stale WORKSPACE_NOT_READY test expectation introduced by the main merge.