feat: worktree-based parallel development for /qq:execute and /qq:commit-push

[Copilot]

Multiple Claude Code sessions can't share one working directory. This adds git worktree support so parallel sessions each get their own filesystem while sharing the .git object database.

skills/execute/SKILL.md

• New --worktree flag creates an isolated worktree + sub-branch before implementation begins
• Section 1 (new): branch detection (blocks on main/master unless --auto), worktree creation with $ORIGINAL_DIR capture, Library/ regeneration warning
• Section 7 (new): merge-back to source branch, cleanup prompt (worktree remove + branch delete)
• Existing sections renumbered 2–6; no behavioral changes to them

skills/commit-push/SKILL.md

• Step 2 (new): detects linked worktree via git worktree list, identifies source branch from naming convention <source>-wt-<name>, resolves main worktree path
• Step 6 (new): offers merge-back + worktree cleanup after push

Not changed

No script changes needed — unity-compile-smart.sh already falls back to batch mode when no Editor is detected, which is the expected path in worktrees.

Example flow

# On branch dev/vehicle-system, three parallel sessions:
/qq:execute damage-system.md --worktree
# → worktree ../project-wt-damage-system on branch dev/vehicle-system-wt-damage-system
# → implements, compiles (batch), tests (batch)
# → merges back to dev/vehicle-system, deletes worktree

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