Upgrade kept issue-reference comments to explicit hyperlinks (// see <url>) #573

Follow-up to the origin-issue-citation cleanup (which removes bare trailing #NNN tags from self-sufficient comments and test titles).

Rationale

The cleanup deliberately keeps issue refs that carry weight — load-bearing tracking refs ("until #X", "remove when #X"), (#NNN review) provenance, and cross-file "decision-shorthand" refs (e.g. #372 = strictReachable-abort). But most of those are still bare numbers, which have low value in the reading context:

• Humans: a bare #526 isn't clickable in a comment or in test output — using it means manually going to GitHub and typing the number. In-source it's also largely redundant with git blame → PR → issue.
• LLMs: a bare number is only useful as a cross-reference join key and is largely redundant with git history; it's actionable only with issue-fetch tooling.

Where a ref is worth keeping, it's worth making actionable — an explicit hyperlink a reader (or agent) can follow directly.

Scope — codify the convention in AGENTS.md

Update the ### Comments subsection (under Coding Standards in AGENTS.md) so the rules match what the cleanup established. Concretely:

• Kept issue/PR refs must be explicit hyperlinks, not bare numbers. Replace the current "Do link load-bearing tracking refs — an issue or PR whose resolution removes or rewrites the commented code" with guidance that such a ref is written as a full URL, e.g. // …rationale. See https://github.com/goosewobbler/releasekit/issues/526 — a bare #NNN is not "linking."
• Drop bare origin/closing-issue citations. State that a trailing #NNN merely tagging the issue a piece of code closed/implemented — where the comment already stands on its own — should be omitted (git history is the canonical trace). This is the rule the cleanup applied.
• Test / describe titles carry no issue refs. A title string can't be a hyperlink, so titles stay pure behavior specs; traceability lives in git, not the title.
• Reconcile the existing "Keep the rationale, drop the citation" line so "citation" clearly covers bare issue numbers, not just drifting details (versions, stack traces).

Scope — the codebase

• Audit the ~140+ deliberately-kept issue refs in comments and, for those that genuinely aid understanding, convert to the linked form above; drop any that add nothing once they'd need a full URL.
• Consider a lightweight lint/check so future refs follow the linked form from the start.

Non-goals

• Not re-litigating the bare-tag removals from the cleanup PR.
• Not adding links to test/describe titles.