chore: add /release-notify skill to credit reporters after a release

[git-nandor]

Summary

• New /release-notify command: works out who reported the issues fixed in a release and drafts a Slack message crediting them by name. Nothing is posted automatically — the block is copy-pasted by a human.
• One gather.mjs script collects the release's Jira tickets and the #instui threads and narrows them down; the command itself decides who the reporter is.
• Reporters are found from two directions, because either can be missing: the Slack link in a Jira description, and channel threads quoting the issue key.
• Adds step 11 to the release process and documents the two read-only credentials (Slack bot token, Jira API token).

Test Plan

• Run /release-notify after a release and check the draft credits the right people. The key thing to eyeball: the reporter must be the thread's root author, not the developer who pasted the Jira link into the thread.
• Needs SLACK_BOT_TOKEN and the three JIRA_* keys in the root .env, and the bot must be a member of #instui — see /slack-setup.
• Already verified against v11.7.4..v11.7.5: of a hand-written reporter list, 4 of 6 matched with no false positives. The other two are unreachable in principle — their report thread never quoted the issue key and their ticket never linked the thread, so nothing connects them.

Fixes INSTUI-5133

🤖 Generated with Claude Code

[github-actions]

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[github-actions]

Visual regression report

Cypress suite: ✅ Passing

Visual diff: ⚠️ Changes detected.

Status	Count
Unchanged	95
Changed	1
New	0
Removed	0

Accessibility (axe): ✅ No violations.

📊 View full report — click a screenshot's ⚠ badge to see each violation boxed on the image, with the offending element named and contrast failures shown as color swatches.

Diff images (1)

badge-canvas.png — 1573 pixels differ

_{Baselines come from the visual-baselines branch. They refresh on every merge to master. The Cypress suite line covers the a11y and console-error assertions — a ❌ there means the suite found real issues even if the visual diff is clean.}

[balzss]

Works, but I think it solves the wrong problem.

We already know the reporter at triage time. /slack-triage resolves the user IDs in Step 1, and puts the thread link in the ticket in Step 5. Then we drop both, and this PR spends a six-month channel scan and 2.5 minutes rebuilding them by guesswork. Your own number: six of 22 tickets had a usable link.

Better: make /slack-triage store the reporter and the thread link as structured data (Jira Reporter field or a reported-by-<slack-id> label), and make it required. Then /release-notify just reads that field and writes the draft.

That means gather.mjs goes away completely, and the Jira credentials in .env.example with it — the atlassian MCP server already gives us Jira. Tickets triaged before this change won't have a reporter, and I'd accept that rather than keep a scanner for them.