feat(app): add open command for existing repo workspaces

[skulidropek]

Summary

• add docker-git open as an alias to open existing project workspaces by repo/issue URL (same project-dir resolution as attach)
• add top-level pnpm open <url> shortcut flow, mirroring existing pnpm clone behavior
• update usage/help/readme and add parser/core tests for open request routing

Verification

• pnpm -C packages/app typecheck
• pnpm -C packages/lib typecheck
• pnpm -C packages/app exec vitest run tests/docker-git/parser.test.ts tests/app/main.test.ts
• pnpm -C packages/lib exec vitest run tests/core/clone.test.ts

Closes #75

[skulidropek]

Retriggering CI on latest commit.

[skulidropek]

Superseded by #82 (same changes from upstream branch so CI can run end-to-end).