Fix: restore projectless new chat startup

[xIGBClutchIx]

Summary

This restores projectless New chat startup in Pocodex by implementing the missing vscode://codex/projectless-thread-cwd bridge contract the current Codex bundle now calls before creating a non-project local thread.

What changed

• add an app-server bridge handler for vscode://codex/projectless-thread-cwd
• return the newer bundle payload shape with cwd, outputDirectory, and workspaceRoot
• resolve projectless threads to homedir() for cwd and outputDirectory while preserving the active existing workspace root for the browser pane
• add focused regression coverage in test/app-server-bridge-basic.test.ts for the projectless thread cwd fetch contract

Root cause

• the current Codex bundle now requests vscode://codex/projectless-thread-cwd when starting a projectless local chat
• Pocodex did not implement that host fetch route, so the request fell through to the generic unsupported-route 501 path
• once that bridge contract failed, projectless New chat startup surfaced Unsupported Codex host fetch URL: vscode://codex/projectless-thread-cwd instead of resolving the thread cwd

Impact

• projectless local chats can resolve their startup cwd again instead of failing on the missing host route
• the current bundle receives the cwd, outputDirectory, and workspaceRoot payload shape it expects
• focused regression coverage now protects this projectless host fetch contract from falling back to unsupported-route behavior again

Validation

• pnpm run check:commit
• pnpm exec vitest run test/app-server-bridge-basic.test.ts
• pnpm dev --listen 127.0.0.1:8891
• live websocket verification against ws://127.0.0.1:8891/session that vscode://codex/projectless-thread-cwd returned 200 with cwd=/root, outputDirectory=/root, and workspaceRoot=/root/repos/xigbclutchix/pocodex