Add --publish: the one ingress control

Everything else in the file gates egress, which left an everyday request unanswered: the
agent starts a dev server inside the sandbox and a human wants to look at it. This puts a
listener on the host that forwards into the container, so a browser, curl or an API
tester reaches the server the sandboxed process runs.

`--publish [ADDR:]HOST:CONTAINER`, config `publish` and AISB_PUBLISH, repeatable, with the
cost named in the flag: it is ingress, so whatever that server serves is reachable from
the address it binds. The default address is 127.0.0.1 because a bare podman -p binds
every interface (measured) -- a port committed in a repo config must not quietly be a port
on the LAN. publish_addr changes the default machine-wide; a spec with a non-loopback
address warns on every run. The grammar is smaller than podman's on purpose: one port per
entry, no ranges, no /udp, no `*` -- this list is reviewed line by line, and typing
0.0.0.0 is the whole warning. An address means three parts, so `127.0.0.1:8000` is refused
rather than guessed. No -p alias: that flag belongs to the harness (claude -p prints).

Fail closed, as elsewhere: --net none cannot forward a port and podman discards the
mapping while starting the container anyway, so "it ran" would mean "nothing is
listening"; a host port already in use is named before the container starts, not after a
build; pick_port() will not hand the gate a port this run publishes. The container is
labelled with the ports it was given, and doctor reports them.

Inside, publish_rules() accepts tcp on the published ports in the input chain and never
the output one. Measured (docs/networking.md): pasta delivers the connection over
loopback, so that rule counts 0 packets on the reference host; it is there for a host or a
bridge network that delivers on the interface, and so that `nft list table inet aisb`
answers "what can reach this container". The ports are validated again inside the
entrypoint, because a ruleset is not somewhere to splice an unvalidated string. And the
measurement that makes a configured preview look broken: a server bound to 127.0.0.1
inside answers inside and refuses the published port outside, so dev servers must bind
0.0.0.0 -- both skills say so in the change that writes the port.

Tests. Unit: the grammar and every port list that looks right and is not, config/env/flag
precedence and dedup, the canonical spec in --print-policy, the `# ingress:` header, the
flags reaching the invocation before the image, egress enforcement byte-identical with a
port open, which chain the rule lands in and that a non-numeric port never reaches nft as
code. e2e (tests/e2e/13_publish.sh): one real container with enforce=nft where the host
gets its 200 and the same run still refuses a domain outside the allowlist at 403, the
server runs as uid 1000 with no capabilities so it cannot widen its own port, and the host
listener is gone when the container exits.

LLM-Generated: true

docs/threat-model.md

@@ -107,6 +107,21 @@ Be honest about these, and design your work accordingly.
    *in the sandbox* by default; `--validate-in host` hands that judgement to you and
    warns on every run.
 
+14. **A published port is a door you opened.** `--publish` (and `publish` in a config
+    file) puts a listener on the host that forwards into the container, so a client
+    outside can reach a server the agent is running. That is the feature — preview a dev
+    server, poke a local API — and it is also the whole risk: whatever that server serves,
+    and whatever it accepts, is reachable from the address it is bound to, by things other
+    than you. The default binds `127.0.0.1`, because a bare podman `-p` binds every
+    interface (measured); a spec with a non-loopback address warns on every run and should
+    not be committed. Egress stays enforced either way: a published port is ingress, not a
+    loosened allowlist, and the same run that answers your browser still refuses a domain
+    outside it (`tests/e2e/13_publish.sh`). The agent inside cannot widen its own port or
+    open a new one — the mapping is decided on the host before the container starts and the
+    command runs with no capabilities. What it *can* do is serve whatever it likes on a port
+    you asked for. Publish the port of the server you meant to preview, and treat that port
+    as published content for as long as the run lasts.
+
 ## Residual risk rating
 
 *Accidental destruction and ambient credential leakage: largely eliminated.*