deploy(althing): post office to 3.6.0 — handle delete/retire verbs go live

The tools had been on 3.6.0 while the container sat on 3.0.0 for seven days, so
the operator verbs failed with "no tool named delete_handle" — they live in the
post office, not the client.

Image built on nh3-dev from a clean tree at 4d26226 and pushed under the
claude-bot namespace; the compose pin moves to the new digest rather than
floating on the tag, since this container is the fleet's whole message bus.

The backup procedure this file documents earned itself again: at stop time the
database was 23.8 MB with a 5.9 MB WAL beside it, so a plain copy would have
produced a database that opens cleanly, smokes green, and is missing the day's
mail. Stop, explicit checkpoint to a zero-byte WAL, copy, then verify counts on
both sides — 76 handles, 995 messages, 1022 recipients, integrity ok.

Post-deploy the same counts came back with handles.retired_at present, and the
memory cap and OOM guard were confirmed by `docker inspect` rather than by
reading the yaml, which is what that file asks for and the only check that can
tell a working cap from a decorative one.

Bus down about four minutes.

stacks/althing-post-office/compose.yaml

@@ -51,7 +51,7 @@ services:
     # Digest-pinned, not tag-floating: `:3.0.0` is a mutable pointer on a registry
     # anyone can re-push, and this container is the fleet's whole message bus. The
     # tag is kept alongside the digest purely so a human can read what it is.
-    image: gitea.phasefinal.com/claude-bot/althing-post-office:3.0.0@sha256:410fed41fa049c41cf83577fd2e49831ea1959ca50b597a8bd35a548e267cf01
+    image: gitea.phasefinal.com/claude-bot/althing-post-office:3.6.0@sha256:13158835488a8ec04f990c97c4f4c68f1d923b12494319cf07392552e68f8a78
     container_name: althing-post-office
 
     # ─── Host networking, so the bind guard keeps working ────────────