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/build-package.sh

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: GPL-3.0-only
 # SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
 # CI package build: cross-compile imsd for aarch64 with crafter-build (the
-# README's "Cross-compiling" flow), run the test suite natively, and package
+# cross-compiling flow of AGENTS.md §4.3), run the test suite natively, and package
 # the result via packaging/APKBUILD.binary. Expects an x86_64 Alpine
 # environment with root — the workflow runs it in an alpine:edge job
 # container on an ordinary runner. Root only installs packages and hands off