feat(happy): support managed workspaces

[Scoteezy]

Summary

Completes native Happy workspace support for Rig machines.

• Advertise worktree support only when managed workspace creation is available.
• Create and bind Happy sessions to deterministic Rig workspaces.
• Return pending workspace preparation before Happy's machine RPC timeout.
• Coalesce concurrent retries and reject reused request IDs with mismatched payloads.
• Expose encrypted workspace listing for the Happy picker, including after credential reloads.
• Validate Happy machine RPC payloads with TypeBox and reject unsafe workspace names.

Validation

• pnpm check
• pnpm --filter @slopus/rig test (3319 tests)
• pnpm format:check
• git diff --check

Companion to the Happy changes in Scoteezy:fix/harden-happy-session-flows.