When users accidentally edit main/dev before starting an agent lane, those changes should follow the new agent branch instead of staying on the protected checkout. agent-branch-start now auto-stashes tracked changes on protected base branches and re-applies them in the created agent worktree.

Constraint: Keep codex-agent launches working even when setup left only untracked files on base branch
Rejected: Auto-transfer all dirty states including untracked-only changes | broke codex-agent tests by forcing unnecessary stash re-apply
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Preserve protected-branch auto-transfer for tracked edits; if stash format changes, keep stash-ref parsing colon-safe
Tested: node --test test/fuzzing.test.js test/install.test.js test/metadata.test.js (pass)
Not-tested: Manual conflict-heavy stash apply with partially overlapping tracked edits across very large repos