fix(agents): external-PR filter used a gh field that does not exist

[lesnik512]

Summary

docs/agents/issue-tracker.md documented the external-PR triage filter as a gh pr list --json
call including authorAssociation. That field does not exist on gh pr list (or gh pr view) —
gh 2.98.0 rejects it outright:

Unknown JSON field: "authorAssociation"

So the documented command fails rather than returning a filtered list. It came in with the stock
setup template and is latent today, since this repo has PRs as a request surface: no — but it
would fail the moment anyone flipped that flag.

Changes

Replaced the line with a REST-API call, where the field exists as author_association (snake_case):

gh api "repos/{owner}/{repo}/pulls?state=open&per_page=100" \
  --jq '.[] | select(.author_association | IN("CONTRIBUTOR","FIRST_TIME_CONTRIBUTOR","NONE")) | {number, title, author: .user.login}'

Verification

Run against this repo: gh api substitutes {owner}/{repo} from the clone, returns
author_association, and the filter correctly yields nothing here — every open PR is authored by a
MEMBER. Dropping the filter returns those rows, confirming the query itself is sound rather than
silently empty.

Same fix applied across the repos carrying this file. chat-app is unaffected — it uses a
local-markdown tracker with no PR section.

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	881	1.000	6.80	911	242	5000	5000	10000
consumer/w1/b100	1085	1.000	6.86	917	243	5000	5000	10000
consumer/w2/b10	1504	1.000	6.77	947	242	5000	5000	10000
consumer/w2/b100	1738	1.000	6.73	952	243	5000	5000	10000
consumer/w4/b10	1362	1.000	6.82	995	261	5000	5000	10000
consumer/w4/b100	1750	1.000	6.86	1035	244	5000	5000	10000
consumer/w1/b100/tfbs100	8682	0.010	6.07	1116	243	5000	5000	10000
producer/w1/b100	2949	0.000	3.04	584	0	0	0	0

Gated (fails the build): delete_calls + tuple counters (upd/del/ins) + the producer's insert_calls, exact; select_calls within +2; wal_records within a 10% band. msg/s, WAL bytes and total calls are informational (timing/FPI noise).