feat(pr-merge): VCS detection (GitHub/GitLab) + ship-stage PR body priority

[iamcxa]

pr-merge mod hardcodes gh CLI and ignores GitLab users. This PR adds VCS detection so the mod works with both GitHub and GitLab remotes.

What changed

• Add VCS detection to startup, idle, and merge hooks (github→gh, gitlab→glab, unknown→warn)
• Replace hardcoded gh pr view/create with VCS-conditional commands
• Add PR body resolution: check for Ship stage PR Draft first, fall back to generic template
• Rename ### PR body template to ### PR body template (fallback) to clarify priority

Review guidance

The VCS detection block (3 lines) is identical across startup/idle hooks — kept inline rather than extracted to avoid forward references in a prose document.

[clkao]

@iamcxa Thanks! Can you split/rebase the PR so it contains only the pr-mod changes?

[iamcxa]

Done — rebased onto clkao/main HEAD (70575809), branch now contains only the pr-merge changes as requested:

• cba2490b feat(pr-merge): VCS detection (gitlab/github) + PR body resolution
• 30d2623e fix(pr-merge): sync docs/plans/_mods/ with mods/ (test requires identical copies)

Files touched: mods/pr-merge.md, docs/plans/_mods/pr-merge.md. Infrastructure commits (prompt-upgrade, skill-binding, workflow-adopt/sync renames) dropped — they're in their own PRs (#119, #120, #130). Ready for re-review.

[clkao]

@iamcxa I am considering if we should ship this, or let the user adopt a github or gitlab specific ones. thoughts?