feat(board,browser): add --repo to view a remote repository's backlog

[ademczuk]

Summary

backlog board --repo <owner/name> and backlog browser --repo <owner/name> render another repository's backlog/ folder without cloning it by hand. Useful for reviewing a teammate's board, checking a dependency's roadmap, or browsing any public Backlog.md project.

How it works:

• The repo is cloned into a local cache (~/.backlog/remotes/<host>/<owner>/<name>) with a shallow, blobless, sparse checkout of only the backlog/ folder. Later runs fetch and hard-reset to refresh.
• Everything downstream is unchanged: the cache dir is handed to the existing Core(projectRoot), so task parsing, the TUI board, and the web UI all work as-is.
• Private repos work through your existing git credentials (the clone reuses your credential helper).
• --ref <branch/tag> picks a ref, --no-refresh uses the cached snapshot offline, BACKLOG_REMOTE_CACHE overrides the cache root.

Read-only intent: the terminal board is read-only by nature. The web UI stays editable, but edits write to the local cache only and are not pushed to the source repo. A web header banner and a terminal line make that clear.

Scope of changes:

• New src/remote/remote-repo.ts: spec parsing (owner/name, https, ssh), cache resolution, clone/refresh.
• src/cli.ts: shared resolveViewRoot() plus the new options on board and browser. With no --repo, the existing local path is untouched.
• src/server/index.ts: optional remoteSnapshot flag exposed at GET /api/remote-snapshot, kept out of /api/config so it never lands in config.yml.
• src/web: RemoteSnapshotBanner component.
• Tests in src/test/remote-repo.test.ts, README and ADVANCED-CONFIG updated.

I am happy to adjust the approach, the flag names, or the read-only behavior if you would prefer something different.

Related Tasks

closes BACK-466

Task Checklist

• I have created a corresponding task in backlog/tasks/
• The task has clear acceptance criteria
• I have added an implementation plan to the task
• All acceptance criteria in the task are marked as completed

Testing

• bunx tsc --noEmit is clean.
• New and touched-area suites pass: src/test/remote-repo.test.ts (8 tests: spec parsing for owner/name, https, ssh forms, cache-dir env override, and a local file:// integration test that clones a seeded backlog and loads it through Core), plus the board and core suites (63 tests, no regressions).
• Verified live against a public Backlog.md project: it clones the backlog, renders the full board, the refresh and --no-refresh paths work, and running board/browser with no --repo behaves exactly as before.

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