docs: match AGENTS.md to the standard's canonical justfile sentence

[lesnik512]

One-line change to AGENTS.md: the justfile sentence now reads "The justfile is the source of truth: just --list, or read it."

This is the canonical wording in Appendix B of the standard after modern-python/.github#133, which FL2 requires word for word. The humanizer passes replaced the original dash in different ways across repos, and this aligns the wording with the standard.

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	1910	1.000	6.71	919	242	5000	5000	10000
consumer/w1/b100	1949	1.000	7.11	1154	243	5000	5000	10000
consumer/w2/b10	2575	1.000	6.72	923	242	5000	5000	10000
consumer/w2/b100	3198	1.000	7.11	1154	243	5000	5000	10000
consumer/w4/b10	2698	1.000	6.73	941	242	5000	5000	10000
consumer/w4/b100	3183	1.000	7.11	1154	243	5000	5000	10000
consumer/w1/b100/tfbs100	9916	0.010	6.11	1130	243	5000	5000	10000
consumer/w1/b100/interval1-10	2111	1.000	7.11	1154	243	5000	5000	10000
producer/w1/b100	6753	0.000	3.04	597	1	0	0	0
producer/w1/b100/ppt1	1586	0.000	4.04	635	0	0	0	0

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