feat(maestro): hot-reload [daemon.policy] via SIGHUP + maestro policy CLI (NAP-237)

`PairAttachVerifier` now stores `allow_inbound` behind `Arc<RwLock<_>>` so
clones share the same set. Two triggers re-read the unified config file
and swap it atomically:

- SIGHUP — `systemctl --user reload maestrod`, or any `kill -HUP`. The
  installer's user unit now sets `ExecReload=/bin/kill -HUP $MAINPID`.
- `POST /api/policy/reload` — used by `maestro policy reload`. The CLI
  reads `[daemon].bind_addr` from the unified config (default 7070) and
  POSTs to `http://127.0.0.1:<port>`. No PID lookup required.

Also adds `maestro policy show [--json]` and `GET /api/policy` for
"did my edit get picked up?" debugging. A broken mid-edit config logs
an error and leaves the previously-effective list in place rather than
locking everyone out.

In-flight attaches keep their existing connection — revocation requires
killing the session, as spec'd.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/INSTALL.md

@@ -126,6 +126,21 @@ Pick `/tmp` if the daemon must outlive your login session and you can't run
 tmux sockets with it). The env var `MAESTRO_DAEMON_TMUX_TMPDIR` takes
 precedence over the file.
 
+## Hot-reloading policy
+
+`[daemon.policy].allow_inbound` (the list of peer daemons allowed to broker-
+attach into this one) can be changed without restarting `maestrod` — restarting
+would kill every live session. Two ways to apply an edit:
+
+```sh
+maestro policy reload     # POSTs to the local daemon's /api/policy/reload
+maestro policy show       # prints the current effective list
+kill -HUP <maestrod-pid>  # equivalent; what `systemctl --user reload` does
+```
+
+Only newly-arriving attach attempts see the change — sessions already
+attached stay attached. To boot a live attacher out, kill the session.
+
 ## What the systemd unit looks like
 
 The wizard writes a *user* unit (`systemctl --user`), since `maestrod` runs
@@ -140,6 +155,7 @@ After=network-online.target
 [Service]
 Type=simple
 ExecStart=%h/.cargo/bin/maestrod
+ExecReload=/bin/kill -HUP $MAINPID
 Restart=on-failure
 RestartSec=5