Shadow mode: Housecarl beside the firewall in force, changing nothing

shadow.enabled in housecarl.yaml loads Housecarl's ruleset so that it
blocks, rejects and translates nothing: drops and rejects count and
accept (an accept only ends Housecarl's chain), policies accept and log
what they would drop, NAT only counts, MSS clamping and the flowtable
are left out, and raw rules that do more than accept, drop or reject are
left out with a warning. Comments mark what would happen ([would drop]).
Filter chains run after the other firewall's (+10) or before (-10); NAT
chains always first. Every change is checked by nftables in both forms.
No probation; takeovers and closing connections are refused. The boot
fallback filters nothing. internal/tablewatch puts the table back after
a flush ruleset. Mode in status, healthz, whoami, the web UI, the log
and the tracer; hc ruleset --enforcing. docs/shadow.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -30,6 +30,10 @@ Go module `housecarl`, `CGO_ENABLED=0`, Linux only for housecarld.
   on probation and rolled back unless confirmed (docs/design.md). An
   empty configuration applies nothing. But never force a rule that keeps
   SSH or the web UI open: the maintainer doesn't want one.
+* **Shadow mode changes no packet** (docs/shadow.md): anything new the
+  renderer writes that drops, rejects, translates or alters packets needs
+  its shadow form (counted, marked `[would …]`) or must be left out
+  there. `TestShadow` fails on what slips through.
 * **Stay within nftables 1.1.3** (Debian 13). The dev machine's newer nft
   accepts things 1.1.3 refuses; the tests with `HOUSECARL_NFT` and the
   e2e test use 1.1.3.