docs: align PR template checklist with the count-free index convention

[taminomara]

Summary

This PR updates the contributor PR-template checklist to stop chasing entity counts that no longer exist, so that the template matches the "indexes stay complete, not counted" convention.

• Rewrites the index-maintenance checklist item: instead of telling contributors to update a count line in CLAUDE.md plus hardcoded counts in docs/concepts.md and README.md, it now asks them to confirm the two indexes, the CLAUDE.md catalog, and docs/concepts.md each list every current skill and agent.
• Drops "counts" from the list of things /han-update-documentation syncs, since there are no longer any counts to sync.
• Scope is two lines in one file. Pure process/documentation text, with no skill, agent, or runtime behavior change.
• Follow-up to a triage that flagged this drift; the adjacent path-fix branch (PR docs: fix stale long-form doc paths after the plugin-rename reorg #80) deliberately left this count-convention drift out of its scope.

What to look at first

• The replacement checklist item (line 20): does it correctly name the four places that must each list every entity, and does the instruction read clearly to a contributor who has never seen the convention? It states the rule prospectively, with no reference to the old count-based wording (that history lives in the commit message, not the checklist).
• Whether the targets it points at are real: the old line referenced a "Counts to verify when editing indexes" line in CLAUDE.md and counts in docs/concepts.md and README.md that have already been removed. Worth a quick confirm that the new wording does not point anyone back at a count that no longer lives in those files.
• The /han-update-documentation paragraph (line 26): removing "counts" should leave the sentence accurate about what that skill still syncs (long-form docs, indexes, cross-references) and not imply it dropped real work.

Files of interest

• .github/pull_request_template.md — the only file changed; two checklist lines realigned to the count-free index convention.