AGENTS.md: document `amend!`, fixup-only PRs, and direct GfW PRs #6628

Job	Run time
config	8s
fuzz smoke test	1m 4s
win+Meson build	4m 3s
documentation	9m 40s
win build	5m 24s
win+VS build (arm64)	4m 40s
win+VS build (x64)	4m 47s
static-analysis	6m 10s
sparse	3m 23s
rust-analysis	1m 20s
osx-meson (macos-14)	23m 57s
osx-gcc (macos-14)	24m 45s
osx-reftable (macos-14)	21m 19s
osx-clang (macos-14)	18m 38s
linux-leaks (ubuntu:rolling)	29m 32s
linux-meson (ubuntu:rolling)	13m 48s
linux-asan-ubsan (ubuntu:rolling)	1m 25s
linux-reftable (ubuntu:rolling)	1m 16s
linux-breaking-changes (ubuntu:rolling)	13m 16s
linux-reftable-leaks (ubuntu:rolling)	25m 46s
fedora-breaking-changes-meson (fedora:latest)	10m 0s
linux-sha256 (ubuntu:rolling)	1m 23s
almalinux-8 (almalinux:8)	10m 16s
linux-TEST-vars (ubuntu:20.04)	13m 3s
linux-musl-meson (alpine:latest)	6m 46s
debian-11 (debian:11)	10m 0s
linux32 (i386/ubuntu:focal)	10m 3s
win+Meson test (6)	11m 44s
win+Meson test (1)	7m 20s
win+Meson test (9)	7m 5s
win+Meson test (4)	7m 25s
win+Meson test (2)	9m 28s
win+Meson test (7)	7m 37s
win+Meson test (0)	6m 59s
win+Meson test (3)	8m 25s
win+Meson test (8)	11m 7s
win+Meson test (5)	9m 52s
win+VS test (8)	8m 9s
win+VS test (9)	10m 42s
win+VS test (1)	5m 36s
win+VS test (6)	6m 29s
win+VS test (4)	7m 5s
win+VS test (2)	9m 17s
win+VS test (7)	6m 3s
win+VS test (5)	8m 58s
win+VS test (0)	8m 2s
win+VS test (3)	7m 21s
win test (8)	8m 11s
win test (5)	10m 25s
win test (6)	6m 39s
win test (9)	10m 28s
win test (0)	6s
win test (1)	6h 5m 0s
win test (4)	7m 29s
win test (7)	6m 48s
win test (3)	8m 10s
win test (2)	7m 48s
	14h 31m 40s

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