Fix workspace path drift after identifier changes

[Octane0411]

Summary

• derive workspace directories from stable issue IDs instead of mutable issue identifiers
• use the stable workspace key for runner setup, runtime cleanup, and dashboard workspace details
• add regression coverage for identifier renames and terminal workspace cleanup

Testing

• pnpm test -- tests/workspace/path-safety.test.ts tests/workspace/workspace-manager.test.ts tests/agent/runner.test.ts tests/orchestrator/runtime-host.test.ts tests/cli/runtime-integration.test.ts
• pnpm typecheck

Closes #46