chore(skills): include pcbjam-shared in the git-feature workflow

The pcbjam repo's own git-feature skills + scripts/git-workflow predated the
web/pcbjam-shared submodule and only covered root+kicad+wxwidgets. Add the MIT
contract repo as a fourth member everywhere:

- repos.sh: pcbjam_shared key (path web/pcbjam-shared, main `main`) + layout doc.
- assert-clean.sh / for-each-repo.sh: count-agnostic wording.
- git-feature-start/commit/sync/finish SKILL.md: pcbjam-shared in the branch /
  commit (kicad, wxwidgets, pcbjam-shared, then root) / rebase (root first) /
  finish orders; root pointer-bump staging + diff now include web/pcbjam-shared,
  and the root stage step excludes it like the other submodule trees.

Mirrors the root pcbjam-private skills, which already covered shared.

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

scripts/git-workflow/assert-clean.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# Exit 0 if all 3 repos are clean (no staged, no unstaged, no untracked files).
+# Exit 0 if all repos are clean (no staged, no unstaged, no untracked files).
 # Exit 1 with a readable message listing the dirty repos otherwise.
 # Untracked files count as dirty - this is intentional: starting a new feature
 # while you have uncommitted new files is almost always a mistake.
@@ -20,7 +20,7 @@ for repo in "${REPOS[@]}"; do
 done
 
 if [ ${#dirty[@]} -eq 0 ]; then
-    echo "All 3 repos clean."
+    echo "All ${#REPOS[@]} repos clean."
     exit 0
 fi