interrupt: add the passive interrupt core

The library holds only the state its own types need when a Ctrl+C
arrives: the interrupted flag flows check to stand down, the cleanup
hook registry for resources that must not outlive the process (the
ephemeral build chroot), and the live view's reporter slot. The
signal handling itself is CLI wiring and lands separately: nothing
here installs handlers, prints or exits, so a library consumer
embedding these types keeps its own signal disposition.

AGENTS.md

@@ -48,6 +48,8 @@ Rules:
   - `prune`, `package_info` — remaining subcommand modules
   - `context` — build contexts: local, ssh, chroot/schroot, unshare
     (`src/context/`)
+  - `interrupt` — Ctrl+C interception and interrupt-time cleanup
+    (`src/interrupt.rs`)
   - `debian` — Debian format primitives: control, versions, checksums,
     arch (`src/debian/`)
   - `apt`, `launchpad`, `distro_info`, `quirks` — archive/distro