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/for-each-repo.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# Run a git command in each of the 3 repos. Echoes before each run.
+# Run a git command in each repo of the 4-repo tree. Echoes before each run.
 # Usage: ./for-each-repo.sh <git-subcommand-and-args>
 #   e.g. ./for-each-repo.sh status --short
 #        ./for-each-repo.sh fetch origin