* fix: prevent parallel worktree path resolution from escaping to home directory
When .gsd is a symlink into ~/.gsd/projects/<hash> (the default layout),
parallel workers resolve their cwd through the symlink. findWorktreeSegment()
then matches /.gsd/ at the user-level ~/.gsd boundary instead of the project
.gsd, causing resolveProjectRoot() to return ~ as the project root.
This corrupts ~/.gsd, creates ~/.git, and crashes pi.
Fix (3 layers):
1. Pass GSD_PROJECT_ROOT env var from coordinator to workers — the
coordinator already knows the real basePath unambiguously.
2. In resolveProjectRoot(), detect when the candidate root's .gsd
matches the user-level ~/.gsd and fall back to reading the worktree's
.git file (gitdir: pointer) to recover the real project root.
3. Existing validateDirectory() already blocks ~ — but the bug bypassed
it because the worktree path itself was 'safe'.
Also fixes the existing test that asserted the buggy behavior as correct.
Closesgsd-build/gsd-2#1676
* fix worktree root resolution for deep symlink paths
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Co-authored-by: Vojtěch Šplíchal <splichal@gmail.com>
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