Retire the old build spec, and point every reference at the architecture (M0.1)

Development Plan M0.1. Every section reference in code, tests, specs/,
the example config, the tuning guide, and the CLAUDE.md and README files
now cites docs/Architecture.md (P, L, D, O, X, Q) instead of the old
spec's A-H, and docs/Architecture-Build-Spec.md is deleted (git keeps
it). The plan's old-to-new table goes with it.

Mostly mechanical, with a per-site choice where one old section split:
A4 became L2 where it meant stop/abandon/the loop and L3 where it meant
approvals; A10 became L6 in the context-budget code and L7 elsewhere; B2
became D4 for outbound Gitea and publishing and D5 for review intake;
'A3 step N' became the matching step of L8's boot sequence. Comments
describing behaviour the architecture drops (planning always escalating,
thresholds tuned at an old build step) now describe the current code and
point at the Development Plan.

docs/Architecture.md gains one line the rewrite missed: intake outages
delay but never fail work (old B2), now in L5, where the poller and
Matrix code cite it.

Comments and docs only; no behaviour change.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/src/nisse_dev/matrix.py

@@ -1,4 +1,4 @@
-"""MatrixTransport (spec A1): Matrix as strictly a UI channel.
+"""MatrixTransport (architecture L5): Matrix as strictly a UI channel.
 
 Implements the Transport interface directly over the Matrix client-server HTTP
 API (token auth, /sync long-poll, /send) with the httpx client already in the
@@ -6,7 +6,7 @@ dependency tree — no Matrix SDK. The surface used is four endpoints; an SDK
 earns its keep when E2E arrives, and bot rooms are created unencrypted until
 then, so encrypted rooms are not supported here.
 
-Inbound handling follows B2's polling posture: on homeserver errors the sync
+Inbound handling follows L5's intake posture: on homeserver errors the sync
 loop backs off (capped) and retries indefinitely — an outage delays reactions
 but never fails anything.
 """
@@ -101,7 +101,7 @@ class MatrixTransport:
     def start(self) -> None:
         """Begin delivering new room messages to the registered callback.
         History from before start() is never delivered (Matrix room history
-        must not become load-bearing context, A1)."""
+        must not become load-bearing context, L5)."""
         if self._callback is None:
             raise TransportError("no message handler registered; call on_message first")
         self._stop.clear()
@@ -184,7 +184,7 @@ class MatrixTransport:
         """Who a leading mention addresses, and the body without a leading
         mention of this agent. Only the start of the message counts: a
         mention later on ("approve, cc @alice") is part of the text. The
-        sender is never read from here; it comes from the event (A4)."""
+        sender is never read from here; it comes from the event (L3)."""
         body: str = content["body"]
         own = self._config.user_id
         if content.get("format") == "org.matrix.custom.html":