docs: update BRANCH LOCK hint to include HEAD -- path form [doc-updater]

[james-in-a-box]

docs: Update BRANCH LOCK agent hint for git checkout HEAD -- <file>

Update sandbox/agent-config/rules/mission.md to reflect the fix from
#2252, which corrected the gateway's is_branch_switch heuristic to
allow git checkout [<tree-ish>] -- <path> (any form with --) as a
file restore rather than a branch switch.

The old hint only mentioned git checkout -- <file>, implying that
git checkout HEAD -- <file> and similar commit-ish forms were not
supported in pipeline sessions. They are now correctly allowed. The
updated hint shows all three forms so agents discover the most useful
one (HEAD or a SHA) rather than only the bare form.

Triggered by: 2fa7dc6 (Fix #2247: allow git checkout HEAD -- <path> in pipeline sessions (#2252))

Test Plan

• Automated: no test changes required (gateway tests in the triggering commit cover the behavior)
• Manual: review that the updated hint text is accurate per the gateway fix

Authored-by: egg

[egg-reviewer]

No agent-mode design concerns. This is a one-line documentation update that improves the accuracy of an existing agent hint, helping the agent discover useful command forms (HEAD -- <path>, <sha> -- <path>) that the gateway already permits. That's orientation, not constraint.

— Authored by egg

[egg-reviewer]

Approve.

Verified the change is accurate against the gateway implementation:

• gateway/git_client.py:1816-1872 (is_branch_switch) explicitly returns False whenever -- is present, regardless of any tree-ish before it. The docstring lists the same three examples the updated hint mentions: git checkout -- file.txt, git checkout HEAD -- file, git checkout main -- file.
• PR #2252 (the cited triggering fix) is merged on main, so the behavior is in production.
• The new hint enumerates HEAD --, HEAD~1 --, and bare --, which is the most useful subset for agents recovering from a wrong-branch commit (the surrounding "Wrong branch fix" line) and for restoring tracked files mid-pipeline.

No issues — wording is accurate, scope is minimal, and the change is self-contained to sandbox/agent-config/rules/mission.md.

— Authored by egg

[james-in-a-box]

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