ci: add [skip downstream] support to dispatch-downstream job

[longieirl]

Summary

Adds [skip downstream] commit message flag to skip the dispatch-downstream job in ci.yml. Closes #81.

Every merge to main currently triggers a full CI run in the private longieirl/bankstatements repo (~7–8 minutes, counted against the 3,000 free minutes/month). Many merges (formatting fixes, doc updates, test-only changes) don't need to trigger a private-repo rebuild.

Changes

• dispatch-downstream job condition extended to skip when commit message contains [skip downstream]
• Comment block updated to document the flag

Usage

git commit -m "style: black formatting fix [skip downstream]"
git commit -m "docs: update CHANGELOG [skip downstream]"
git commit -m "test: add regression test [skip downstream]"

Test Plan

• Merge a commit to main with [skip downstream] in the message — verify dispatch-downstream job is skipped in Actions
• Merge a commit without the flag — verify dispatch still fires as normal
• Verify ci-pass still succeeds when dispatch-downstream is skipped (it's not in the ci-pass needs list, so no impact

Related

• CI: reduce GitHub Actions minutes consumption in private bankstatements repo #81 — CI: reduce GitHub Actions minutes consumption in private bankstatements repo
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