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/approvals.py

@@ -1,4 +1,4 @@
-"""Plan externalisation and Gitea-backed approvals (spec A4, Part H).
+"""Plan externalisation and Gitea-backed approvals (architecture L3, D2).
 
 Plans are posted as comments on a per-project Gitea issue. The approval
 comment — its author, timestamp, and revision tag — is the durable off-box
@@ -7,7 +7,7 @@ pointer to it, so boot reconciliation can re-derive gate state from Gitea
 even after the state volume is lost: external truth wins.
 
 Approvals arriving here bind to a revision exactly as they do over transport
-(A4). The first valid approval on any channel wins; the orchestrator echoes
+(L3). The first valid approval on any channel wins; the orchestrator echoes
 the outcome to the other channel so the records converge.
 """