README, AGENTS.md: a short README for people, the detail in AGENTS.md

The README had grown to 422 lines: the device and carrier table came
after 61 lines of architecture, and the configuration section after 237
lines of architecture, that table, build, run and packaging notes and
GNOME Calls detail.

AGENTS.md, a file for coding agents, takes that detail and adds to it: a
reference to the tree at this commit, written from the code and the
bench record. The README is now what a person needs: what imsd is and
its status, the verified device/carrier table, what it needs, how to
install and enable it, the two dialers, a status check for the installed
daemon, a `Not there yet` list, and where the rest is.

In-tree comments that sent readers to README sections which moved — the
cross-compiling flow (the CI workflow, build-package.sh,
APKBUILD.binary), GNOME Calls' known gaps (imsd-ofono-restart-calls.sh),
the D-Bus ABI (main.cpp) and the Configuration section (imsd.service) —
point at AGENTS.md's sections now.

implementations/main.cpp

@@ -6,7 +6,7 @@
 imsd — the userspace IMS/VoLTE daemon (control plane).
 
 Owns net.catcrafts.IMS1 on the system bus with the project's frozen D-Bus ABI
-(README) and drives it with the engine core: register on the IMS PDN (USIM AKA
+(AGENTS.md §8.1) and drives it with the engine core: register on the IMS PDN (USIM AKA
 + IPsec sec-agree, fresh or warm-resume, with a keepalive re-REGISTER refresh),
 run outgoing and incoming calls through the Imsd:Engine call machine (inbound
 INVITEs on the protected-port listeners become ringing calls a dialer answers