feat: add managed Secure Enclave commit signing

[robbie-c]

Summary

• add an opt-in macOS Secure Enclave signing broker and bundled ssh-keygen compatibility wrapper
• add Settings → Commit signing to enable managed signing, display the generated public key, copy it, and open GitHub's SSH key settings
• persist enablement, configure future local Claude/Codex sessions, and cleanly restore the prior Git/SSH environment when disabled
• sign the native helper with APPLE_CODESIGN_IDENTITY in local builds and show a development-only local code-signing prerequisite when entitlements are unavailable
• document setup, behavior, security boundaries, and migration from Secretive

Security boundary

• pass the broker control secret over stdin instead of process arguments
• accept control requests only from the workspace-server parent process using the Unix peer PID
• authorize SSH signing only for registered agent processes and their descendants
• stop newly spawned agents until registration completes, resume them on success or failure, and revoke access after process exit
• retain one random broker-lifetime SSH socket path to avoid concurrent-session environment races

Test plan

• pnpm --filter @posthog/workspace-server exec vitest run src/services/signing-access/service.test.ts src/services/agent/agent.test.ts (36 tests)
• pnpm --filter @posthog/workspace-server typecheck
• pnpm --filter @posthog/code typecheck
• native helper compilation with the production framework flags
• pnpm --filter @posthog/code compile
• node scripts/check-host-boundaries.mjs
• pre-commit pnpm typecheck

QA

• found and fixed an immediate-exit race that could register a PID after its revoke request
• found and fixed a failed-SIGSTOP path that could authorize an unstopped process
• verified both cases with focused regression tests

Local development

• Secure Enclave key persistence requires configured Apple code signing; ad-hoc signatures fail with OSStatus -34018

Notes

• add the displayed public key to GitHub twice: as an Authentication Key and a Signing Key
• remote skills downloads emit non-fatal offline warnings during local Electron builds when GitHub cannot be resolved

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit f0da6b0.}