release skill: add the release-notes step (generated list, ledger, known issues, reporter thanks) and the re-cut procedure

[cliffhall]

Problem

The release skill's step 3 says to "generate the notes and publish" in the GitHub UI, and nothing more. The v2.9.0 release established a fuller release-notes format, and it lives only in that release and in the conversation that produced it:

• the generated What's Changed list
• the smoke-ledger line
• a Known issue(s) section when there are any
• a "Thanks for helping us improve" section that credits the community members whose issues the release addresses. GitHub adds everyone @-mentioned in the body to the release's Contributors strip, so these reporters appear there too.

It also taught two things the skill does not say:

• A release event runs the workflow from the tag's commit. Deleting a Release in the UI leaves its tag behind, so a re-cut must delete the tag too.
• Editing a published release's notes is safe: every tag's main.yml triggers only on release: types: [published].

Change

Add a release-notes step to the release skill's step 3, with a recipe tested against 2.9.0 that reproduces its published Thanks section exactly:

• releases/generate-notes for the list
• the ledger line and known issues appended
• the reporter credit: the authors of the issues each PR closes, minus maintainers (admin / maintain / write) and bots, with the lead-in "This release addresses issues…"

Also add a short "If the release run fails" subsection covering the tag-commit rule and the re-cut steps.

Scripting all of this into a tested helper, and creating the Release from it, remains #2550.

[cliffhall]

Triage: Priority Medium (total 8)

• Severity 2 — a manual step that lives only in memory; there is an easy workaround
• Urgency 4 — wanted before the next release (v2.10.0)
• Bonuses: +1 milestoned (v2.10.0), +1 assigned

Board: #28, Status In Progress.