resolveWorktreeRoot() unconditionally resolved linked worktrees to the main
repo root. When a linked worktree has its own independent .planning/ directory
(e.g., Conductor workspaces), all GSD commands read/wrote the wrong planning
state. Add an early return that checks for a local .planning/ before falling
through to main repo resolution.
The caller in gsd-tools.cjs already had this guard (added in #1283), but the
function itself should be correct regardless of call site. This is defense-in-
depth for any future callers.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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