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.

packaging/imsd-ofono-restart-calls.sh

@@ -3,8 +3,8 @@
 # SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
 #
 # ExecStartPost of imsd-ofonod.service. GNOME Calls 50.0 keeps a stale modem
-# when org.ofono vanishes and dials nothing until it is restarted (README,
-# Known gaps), so every start of imsd-ofonod after org.ofono went away
+# when org.ofono vanishes and dials nothing until it is restarted (AGENTS.md
+# §8.3, Known gaps), so every start of imsd-ofonod after org.ofono went away
 # restarts the Calls daemon of each user running one. Its first start since
 # boot is not one: no Calls has seen org.ofono go, and a Calls that runs
 # already (a login during the bring-up) is left alone. The unit's own