fix(context-restore): prefer the current branch's own checkpoint (#2052)

All worktrees of a repo share one origin-derived slug, so they share one
`~/.gstack/projects/<slug>/checkpoints/` dir. `/context-restore` loaded the
newest checkpoint across the whole dir, so in one worktree it could silently
restore a *sibling worktree's* newer checkpoint.

Step 1 now orders candidates current-branch-first (read from each file's
`branch:` frontmatter), keeping other branches as a fallback. A branch is
checked out in at most one worktree, so this stops cross-worktree contamination
while preserving Conductor cross-branch handoff: when the current branch has no
checkpoint of its own, the full newest-first set is still used.

- scan the 200 newest before partitioning so a current-branch checkpoint sitting
  below a burst of sibling saves is still found; output still capped at 20
- non-git / detached HEAD / branchless legacy saves fall back to the old
  newest-first behavior (back-compat)
- +5 regression tests in context-save-hardening.test.ts (the #2052 bug case
  fails on the old pipeline); regenerated SKILL.md + proactive-suggestions.json

Fixes #2052

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/proactive-suggestions.json

@@ -40,7 +40,7 @@
     },
     "context-restore": {
       "lead": "Restore working context saved earlier by /context-save.",
-      "routing": "Loads the most recent\nsaved state (across all branches by default) so you can pick up where you\nleft off — even across Conductor workspace handoffs.\nUse when asked to \"resume\", \"restore context\", \"where was I\", or\n\"pick up where I left off\". Pair with /context-save.\nFormerly /checkpoint resume — renamed because Claude Code treats /checkpoint\nas a native rewind alias in current environments.",
+      "routing": "Loads the most recent\nsaved state (preferring the current branch, falling back across branches) so\nyou can pick up where you left off — even across Conductor workspace handoffs.\nUse when asked to \"resume\", \"restore context\", \"where was I\", or\n\"pick up where I left off\". Pair with /context-save.\nFormerly /checkpoint resume — renamed because Claude Code treats /checkpoint\nas a native rewind alias in current environments.",
       "voice_line": null
     },
     "context-save": {