docs: scrub #NNN self-references (closes #719)

Last pass of the docs-from-code → docs/ epic (#708). Drops every
attribution cookie from docs/ + README.md + CLAUDE.md so the
source-tree files no longer reference the issue tracker. Issue
threads + commit history retain the references — those are the
canonical record.

- README.md: drop #701 / #660×2 / #551 from matrix + display-name
  sections, rephrase to convey the semantics directly
- CLAUDE.md: scrub 18 cookies from the file map (#655, #15, #784,
  #832, #444, #425, #361, #548, #598, #539, #544, #589, #701,
  #658, #280, #660, #551, #764, #772, #793, #14, #805)
- docs/agent-hierarchy.md: drop #658 ×3 (per-agent user is the
  current shape, not a transition)
- docs/conventions.md: drop #571 (replaced with a docs xref to
  persistence.md::matrix-avatar-sync)
- docs/gateway.md: scrub vhost-map table cookies + Sub-domain
  rationale + Per-agent unix-socket upstream + Self-signed TLS +
  Firewall posture + HIVE_FORGE_URL + Per-agent error pages
  sections; drop the trailing 'Sequencing history' issue list +
  the 'Next-up' issue-link footnote
- docs/matrix.md: scrub serverName/gatewayHost + Default-closed
  firewall + Provisioning flow + Initial rollout + Assertion
  rationale + fluffychat-web build fixes; drop the trailing
  'Sequencing history' issue list
- docs/network.md: drop 'Why ship before #14' #805 quote +
  Container shape #805 attribution + trailing 'Sequencing history'
  + Cross-references issue links; rename v2 column to 'after netns
  isolation'
- docs/web-ui.md: drop #784 from Container row, replace with a
  docs xref to docs/gateway.md::Per-agent unix-socket upstream

Only remaining #NNN in docs/ is the literal markdown-heading
example in docs/forge.md (`#tag`, `#123`, `#!/bin/bash`)
which demonstrates the renderer's behaviour — not an attribution
cookie.

README.md

@@ -112,7 +112,7 @@ the wider swarm human-readable display labels. Distinct from
 the dashboard chrome and the per-agent system prompt; the domain is
 how things are addressed on the wire. Federated hives at different
 domains can share a swarm name. Both default to null — chrome falls
-back to the domain, the prompt simply doesn't mention them (#701).
+back to the domain, the prompt simply doesn't mention them.
 
 Optional: set `services.hyperhive.matrix.enable = true;` to spin up a
 private [matrix-tuwunel](https://github.com/matrix-construct/tuwunel)
@@ -120,16 +120,17 @@ homeserver in a nixos-container. Requires either
 `services.hyperhive.domain` or `services.hyperhive.matrix.serverName`
 to be set (eval fails with a clear error if both are absent). The
 `server_name` (embedded irrevocably in every user and room ID)
-defaults to `<domain>` (bare hive domain, since #660); the hive-gateway serves
+defaults to `<domain>` (bare hive domain); the hive-gateway serves
 `.well-known/matrix/{client,server}` endpoints so clients auto-discover the
 homeserver without a subdomain. **Existing homeservers** must set
 `services.hyperhive.matrix.serverName = "matrix.<domain>";` explicitly to
-preserve pre-#660 user/room IDs. Override with
+preserve their existing user/room IDs (the `server_name` is embedded
+into every ID). Override with
 `services.hyperhive.matrix.serverName = "chat.example.com";` for a custom
 server name. State
 lives at `/var/lib/nixos-containers/hive-matrix/`. Federation is
 enabled with an empty `trusted_servers` list; e2ee is deferred to
-a follow-up (#551).
+a follow-up.
 
 The hive-gateway nginx container serves a
 [fluffychat-web](https://fluffychat.im) matrix client at `/matrix/`