chore: reference internal actions/* via $/, not a version pin

[ineedjet]

Summary

• collect-issues-shared.yml, collect-pull-requests-shared.yml, and proposal-shared.yml referenced their own sibling composite actions (actions/collect-issues, actions/collect-pull-requests, actions/proposal, actions/telegram-notify) via explicit version pins (@v0.5, @v0.6) or @main — requiring a manual bump every release, which is exactly the churn Dependabot's chore(deps): bump rubykatzen/starcast from 0.2 to 0.8 #81 was chasing
• rubykatzen/baseline's own shared workflows (e.g. lint-shared.yml) instead reference sibling actions via $/, which resolves to "this repository, at the same ref the caller pinned for the shared workflow itself" — confirmed working today, since starcast already calls lint-shared.yml@v0.18.1 successfully and it internally uses $/.github/actions/... throughout (see https://github.com/orgs/community/discussions/111928 for background on the syntax)
• Adopting the same pattern here means these internal references can never drift out of sync with a release and never need a version bump again — closing this class of Dependabot PR for good
• Also drops the now-stale comment on actions/proposal's old @main pin, since there's no longer a tag to "switch back to"

Supersedes #81 (closing that PR separately, since it just bumped the old pins to v0.8 rather than removing them).

Test plan

• pre-commit hooks (yamllint, pymarkdown, actionlint) pass on the commit
• Exercise proposal-shared.yml's apply/reject/propose jobs (or collect-issues-shared.yml/collect-pull-requests-shared.yml) end-to-end from a real caller to confirm $/ resolves as expected in production, not just in lint-shared.yml