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.

docs/conventions.md

@@ -350,8 +350,8 @@ Shape contract — every one of these:
    `systemctl restart` gymnastics).
 5. **Re-runnable**: a second invocation produces the same final
    state (idempotent uploads, idempotent config rewrites). Used
-   by the `.path` watchers that re-fire on token appearance
-   (#571 — see `docs/persistence.md::matrix-avatar-sync`).
+   by the `.path` watchers that re-fire on token appearance (see
+   `docs/persistence.md::matrix-avatar-sync`).
 
 The artefact lives under the agent user's home where applicable
 (`~/.config/tea/config.yml`) and is chown'd to that user, but the