After milestone completion and merge, the process cwd could remain
inside .gsd/worktrees/<MID>/, causing new milestone writes to land
in the wrong directory.
Three-layer fix:
1. escapeStaleWorktree() at startAuto entry — detects if base path
is inside .gsd/worktrees/ and chdir back to project root
2. stopAuto() unconditionally restores cwd to originalBasePath,
not just when isInAutoWorktree returns true (module state may
have been cleared by mergeMilestoneToMain already)
3. Milestone merge error handler restores cwd on partial failure
where mergeMilestoneToMain chdir'd but then threw
Closes#608
7e25e6d427