Fix/2329 workspace default branch

[ravk1234]

Closes #2329

Summary

• Detect each workspace child repository’s default branch during registration, preferring origin/HEAD over the checked-out branch.
• Persist it in a new additive workspace_repos.default_branch migration.
• Expose defaultBranch through the project API, CLI DTO, OpenAPI specification, and frontend types.
• Add regression coverage for persistence, detection, and API/CLI responses.

Scope

Child worktree materialization is not present on main. This PR stores the branch metadata for that lifecycle path to consume later.

Workspace import validation, SCM observation, and session cleanup behavior remain unchanged.

Validation

• go test ./internal/httpd/controllers ./internal/httpd/apispec ./internal/httpd/apispec/specgen ./internal/cli ./internal/service/project ./ internal/storage/sqlite/store -count=1
• npm run frontend:typecheck
• `npm run api