source-control: three deferred P2 review findings from PR #72 (round-10)

[kyle-sexton]

Deferred at merge time of #72 (classified VALID (defer) — merged with threads resolved per the EPIC melodic-software/medley#1273 merge protocol). Each is a small doc/logic hardening in the source-control plugin:

1. create.md §2.4.0 — require the issue to be OPEN before emitting Closes #N (codex r3564614067). gh issue view succeeds on closed issues; check .state == "OPEN" (else fall through to the orphan-PR prompt) so a stale branch number cannot ship a misleading auto-close line.
2. babysit.md §5.1.2 — merge-commit branches must integrate via merge, not rebase (codex r3564614069). The snippet always rebases; add a git log --merges origin/$DEFAULT_BRANCH..HEAD pre-check and take git merge origin/$DEFAULT_BRANCH + plain push on merge-workflow branches, matching the prose rule below the snippet.
3. merge.md §4.3 — dirty-tree guard on the worktree-reuse path (codex r3564614071). The regular-branch path stashes uncommitted changes before checkout; the worktree-reuse git checkout -b path needs the same git status --porcelain guard so local edits don't silently carry onto the next task branch.