Add --ref support to repo checkout

[princepal9120]

This adds an optional --ref flag to multica repo checkout so an agent can check out a branch, tag, or commit directly instead of always starting from the repo default branch.

That matters for review and QA flows where the target revision is already known. Without this, the agent has to mutate the checked-out worktree after the daemon creates it, which is fragile and can leave the daemon's repo metadata out of sync.

What changed:

• multica repo checkout <url> --ref <branch-or-sha> now passes the requested ref to the daemon.
• The repo cache resolves branch names, tags, and commit SHAs before creating the worktree.
• Runtime instructions mention --ref for review/QA tasks.
• Added coverage for branch refs, commit refs, and unknown refs.

Fixes #1869.

This also helps #1898 because agents can request the exact branch or commit up front instead of running a separate git checkout inside a daemon-owned worktree.

Validation:

go test ./internal/daemon/repocache ./cmd/multica ./internal/daemon ./internal/daemon/execenv

[vercel]

@princepal9120 is attempting to deploy a commit to the IndexLabs Team on Vercel.

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