fix(native): resolve missing Git worktree refs

[tlgimenes]

Summary

• replace version-specific Git stderr matching with structural ref resolution
• resolve worktree start points to immutable SHA-1 or SHA-256 commit OIDs
• repair missing or dangling origin/HEAD and prevent nonempty remotes from silently becoming orphan branches
• preserve upstream configuration, cancellation, and collision handling
• make sandbox setup failures neutral across repository access, branch selection, and local Git configuration
• document the incident, invariants, implementation, and verification plan

Root cause

The missing-branch fallback depended on two human-readable errors from git worktree add. The reporter Git version emitted Not a valid object name instead, so Studio returned immediately and never tried origin/HEAD even though clone and fetch had succeeded.

Testing

• bun run fmt
• cargo fmt --all -- --check
• cargo test -p local-api setup::clone::tests:: -- --test-threads=1 (18 passed)
• cargo test -p local-api sandbox::manager::tests:: -- --test-threads=1 (29 passed)
• cargo clippy -p local-api --all-targets -- -D warnings
• cargo test --workspace (1,085 passed; 2 ignored)
• pre-commit formatting and workspace clippy hooks

---

Summary by cubic

Resolve Git worktree start points structurally and pin them to immutable commit OIDs. This restores missing-branch fallback across Git versions, prevents orphan branches on nonempty remotes, and makes worktree creation robust to concurrent fetch/prune.

• Resolve refs/remotes/origin/* via rev-parse^{commit}; confirm absence with show-ref; repair origin/HEAD using remote set-head -a; allow no start point only when rev-list --max-count=1 --remotes=origin is empty. Always pass the resolved OID to git worktree add.
• Handle collisions: if canonical holds the branch, detach and retry with the same OID; if a sibling worktree holds it, create a detached worktree at that OID. Preserve branch.<name>.{remote,merge}.
• Change behavior: a nonempty remote without a resolvable default now fails with guidance instead of creating an orphan; cancellations remain cancellations.
• Update UX: neutralize failure copy in SandboxManager and tweak the completion trace.
• Expand real-Git tests: SHA-1/256 OID parsing; requested-branch precedence over origin/HEAD; missing/dangling origin/HEAD repair; sibling-held branch fallback reusing the same OID; nonempty-remote-without-default failing early; cancellation classification; orphan behavior guarded by rev-list.
• Maintenance: satisfy Rust 1.98 clippy in log_store with as_chunks::<2>(); remove the implementation plan doc.

^{Written for commit 880f985. Summary will update on new commits.}