docs: restructure into topic subdirectories, collapse duplicated index

Per mara's go-ahead on hyperhive#3902 ("getting started is good, but
terminal rendering does not go in there i think"):

Moved 21 top-level docs/*.md files into 7 new topic subdirectories
(existing web-ui/, turn-loop/, swarm/, tools/, crates/ untouched):
  getting-started/  setup.md
  agent-lifecycle/  agent-hierarchy.md, approvals.md, persistence.md
  trust-boundary/   boundary.md, security.md
  integrations/     forge.md, matrix.md, github.md, knowledge.md
  networking/       gateway.md, network.md, snapshot-store.md
  scheduler/        jobq.md, coordinator.md, ci.md, observability.md
  process/          conventions.md, gotchas.md, pr-review-gate.md
  web-ui/           terminal-rendering.md (moved into the EXISTING dir,
                    per mara's correction to the original getting-started
                    guess -- it's UI implementation detail, not onboarding)

The physical layout now matches docs/README.md's own topical headers,
which already amounted to this taxonomy -- see the scoping comment on
the issue for the two findings that motivated this (a genuine
duplication between CLAUDE.md's old "Reading paths" list and
docs/README.md's grouped one, since drifted out of sync with each
other; and the flat layout not matching the grouping we already had).

Fixed every cross-reference this moved across the whole repo (~120
files: docs/ internal links at every depth, Rust doc comments, nix
module option docs, crate READMEs) -- verified two ways: a grep sweep
confirming zero remaining references to any old path, and a script
that resolves every markdown link in docs/**/*.md + CLAUDE.md +
README.md against the filesystem and reports anything that doesn't
exist (zero broken links).

Collapsed CLAUDE.md's "Reading paths" section (the duplicate) down to
a pointer at docs/README.md, now the single index. Rewrote
docs/README.md itself to use the new subdirectory paths and added the
one doc it was missing that CLAUDE.md's old copy had (pr-review-gate.md).

Classified all 22 docs/*.md files first via a haiku subagent (mara's
suggestion) on two axes -- proposed grouping and operator-vs-
implementation focus -- before finalizing the taxonomy; spot-checked
the report and found internal inconsistencies (its classification
table disagreed with its own summary section for a few files), so this
taxonomy is my original proposal + the one correction mara gave
directly, not a blind application of the subagent's table. The
operator-focus data it gathered is still useful for a follow-up
content pass (docs skewing 'mixed' rather than pure operator-facing),
not addressed in this PR -- structure only.

nix fmt clean, both pre-push lints clean.

nix/host-modules/hive-forge/default.nix

@@ -107,7 +107,7 @@ in
   # Private Forgejo in a `hive-forge` nixos-container, shared host
   # netns. Agents reach it at `forge.<domain>` via the gateway. State
   # at `/var/lib/nixos-containers/hive-forge/var/lib/forgejo/` survives
-  # restart. See `docs/gateway.md::hive-forge container shape`.
+  # restart. See `docs/networking/gateway.md::hive-forge container shape`.
 
   # External Forgejo/Gitea/Codeberg-compatible forges (beyond the mandatory
   # internal one) are entirely dashboard-provisioned — no nix config here.
@@ -179,7 +179,7 @@ in
         Public hostname for the forge. Doubles as both the forgejo
         `DOMAIN` setting (clone URLs forgejo advertises) AND the
         gateway vhost server-name when `behindGateway = true`
-        (sub-domain routing — see `docs/gateway.md`).
+        (sub-domain routing — see `docs/networking/gateway.md`).
 
         Defaults to `forge.''${services.hyperhive.swarm.domain}` — the
         swarm's domain, not this hive's, because a swarm runs **one**
@@ -251,7 +251,7 @@ in
       description = ''
         Serve forgejo through the hive-gateway nginx as a sub-domain
         vhost (`server_name = cfg.domain`) instead of directly on
-        `httpPort` (sub-domain routing — see `docs/gateway.md`).
+        `httpPort` (sub-domain routing — see `docs/networking/gateway.md`).
 
         When `true`:
         - The gateway adds a `server { server_name = ''${cfg.domain}; }`
@@ -473,7 +473,7 @@ in
     # `server_name = forge.domain`, proxies all `/` → forgejo. Tuned for
     # git: `client_max_body_size 1G`, `proxy_read_timeout 1h` (multi-GB
     # clones). SSH stays direct on `forge.sshPort`. See
-    # `docs/gateway.md`.
+    # `docs/networking/gateway.md`.
     services.nginx.virtualHosts = lib.optionalAttrs cfg.behindGateway {
       "${cfg.domain}" = (gatewayCfg.lib.tlsFor cfg.domain) // {
         listen = gatewayCfg.lib.listen;