Fix subagent navigation to parent workspace

[lil-goat]

Summary

• Resolve a subagent's workspace from its parent agent when the child cwd is not a known workspace directory
• Keep the existing host-agent fallback for missing/unknown agents
• Add a regression test for subagent composer navigation into the parent workspace

Closes #1130

Testing

• npm run test --workspace=@getpaseo/app -- src/utils/navigate-to-agent.test.ts
• npm run format:check:files -- packages/app/src/utils/navigate-to-agent.ts packages/app/src/utils/navigate-to-agent.test.ts
• npm run lint -- packages/app/src/utils/navigate-to-agent.ts packages/app/src/utils/navigate-to-agent.test.ts
• npm run build --workspace=@getpaseo/relay
• npm run build --workspace=@getpaseo/server
• npm run typecheck --workspace=@getpaseo/app
• pre-commit hook: lint, format, and npm run typecheck --workspaces --if-present