docs: fold the root docs/ tree into the Docusaurus site and delete it

docs/ was not the documentation site; it was a grab bag of long-form
design notes, wire contracts and observability guides that landed with
feature PRs because their authors needed somewhere to put them. Root
AGENTS.md already says long-form dev docs live in
website/docs/developer-guide/; this moves the 14 living documents there
(or to the matching user-guide section) so they are published, searchable
and linked from the sidebar instead of being found by grep only.

Developer guide: micro-compaction, gateway-session-lifecycle (was
session-lifecycle), state-db-recovery, multiplexing-gateway,
chronos-managed-cron-contract, relay-connector-contract, observer-hooks
(was observability/README), gateway-monitoring (observability/monitoring),
relay-shared-metrics, middleware, streaming-tts, billing-lifecycle.
User guide: egress/network-isolation (was security/network-egress-
isolation), features/kanban-multi-gateway (was kanban/multi-gateway).

Each page got title/description frontmatter and a sidebar entry; repo-
relative links became site links or GitHub blob URLs; two MDX brace
hazards escaped. Every in-tree pointer (module docstrings, config
comments, the relay conformance test's Path, the monitoring-doc test,
gateway-internals, cron-internals, kanban docs, .dockerignore, AGENTS.md)
now names the new location. `docusaurus build` passes with no unresolved
links on the moved pages.

tests/monitoring/test_cron_health_export.py

@@ -88,7 +88,7 @@ def test_registered_observable_metric_names_cover_snapshot_metrics(monkeypatch):
     observable-gauge metric_names list, or the OTLP exporter never observes it.
 
     This asserts the vocabulary-registration invariant documented in
-    docs/observability/monitoring.md: an emitted-but-unregistered gauge is
+    website/docs/developer-guide/gateway-monitoring.md: an emitted-but-unregistered gauge is
     silently dropped. Regression guard for background_work / cron additions.
     """
     from agent.monitoring import gateway_health_export
@@ -128,7 +128,7 @@ def test_registered_observable_metric_names_cover_snapshot_metrics(monkeypatch):
 def test_monitoring_docs_distinguish_relay_health_scope_and_terminal_flush():
     from pathlib import Path
 
-    text = Path("docs/observability/monitoring.md").read_text(encoding="utf-8")
+    text = Path("website/docs/developer-guide/gateway-monitoring.md").read_text(encoding="utf-8")
 
     assert "Hermes Agent-owned Relay transport health" in text
     assert "authoritative shared connector/platform state" in text