BBC2-19 add bb repo view

[b2l]

Summary

• New bb repo view [workspace/repo] that fetches a single repository via GET /repositories/{workspace}/{repo_slug} and shows name, owner, visibility, default branch, primary language, creation/update timestamps, URL, and description.
• Defaults to the auto-detected repo; a positional argument overrides. -R is also accepted for consistency. If both are passed, the positional wins — matches how gh repo view behaves.
• Adds a new src/backend/repositories/ module with the Repository shape and RepositoryError class, mirroring the pullrequests backend pattern.
• The generated `repository` schema nests optionals several layers deep; `toRepository` flattens to a surface the renderer can consume without guards.

Test plan

• `bun test` — 160 tests pass (5 new backend tests: full mapping, owner fallback display_name → nickname → username, missing-fields defaults, 404, 403)
• `bun run lint` clean
• `bunx tsc --noEmit` clean
• Manual: `bb repo view --help` shows correct help

Manual test path

1. `bb repo view` on a checked-out Bitbucket repo → prints the current repo's metadata.
2. `bb repo view some-workspace/some-repo` → prints that repo's metadata (ignoring the current checkout).
3. `bb repo view -R some-workspace/some-repo` → same result via the flag form.
4. `bb repo view some-ws/some-repo -R different-ws/different-repo` → positional wins; prints `some-ws/some-repo`.
5. `bb repo view nonexistent-ws/nope` → clear 404 error, exit 1.
6. `bb repo view private-inaccessible/repo` (one you don't have access to) → clear 403 error, exit 1.
7. `bb --json repo view` → JSON object with all fields.
8. Run outside a git repo without `-R` or positional → `Not inside a git repository` from resolveRepository.

Closes BBC2-19.