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/tests/test_approvals.py

@@ -1,5 +1,5 @@
 """Plan externalisation to Gitea and approvals arriving on either channel
-(spec A4, Part H)."""
+(architecture L3, D2)."""
 
 import io
 from pathlib import Path
@@ -96,8 +96,8 @@ def test_project_start_opens_an_issue_and_posts_the_plan(tmp_path: Path) -> None
     issue, body = gitea.comments[0]
     assert issue == 1
     assert "plan 1" in body
-    assert "approve rev-1" in body  # the tag is printed for copy-paste (A4)
-    # The store keeps a pointer, not the plan record itself (Part H).
+    assert "approve rev-1" in body  # the tag is printed for copy-paste (L3)
+    # The store keeps a pointer, not the plan record itself (D2).
     assert store.get_state().data["board"]["issue"] == 1
 
 
@@ -136,7 +136,7 @@ def test_non_operator_gitea_comment_cannot_approve(tmp_path: Path) -> None:
 
 def test_agents_own_comment_cannot_approve_its_plan(tmp_path: Path) -> None:
     # The agent posts plans as nisse-dev-testing; that login is not on the
-    # operator allowlist, so its own comments can never satisfy its gate (A4).
+    # operator allowlist, so its own comments can never satisfy its gate (L3).
     gitea = FakeGitea()
     orch, store, transport, _ = build(tmp_path, gitea)
     transport.receive("don", "use demo")