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/agent-modules/forge.nix

@@ -180,7 +180,7 @@ in
 
     # One-shot: tea config.yml from the seeded forge token. Shape
     # contract (always exit 0, no set -e, skip-silently, re-runnable):
-    # docs/conventions.md::Best-effort oneshot services.
+    # docs/process/conventions.md::Best-effort oneshot services.
     # Not generated at all when no forge is configured: an absent
     # integration rather than one pointed at a guessed address.
     systemd.services.tea-login = lib.mkIf (config.hyperhive.forge.url != null) {
@@ -251,7 +251,7 @@ in
     # service fires too early and exits with "no forge-token found".
     # Without this path unit, RemainAfterExit=true would prevent systemd
     # from ever re-running the service. See
-    # docs/persistence.md::forge-avatar-sync.
+    # docs/agent-lifecycle/persistence.md::forge-avatar-sync.
     systemd.paths.forge-avatar-sync = lib.mkIf (config.hyperhive.icon != null) {
       description = "trigger forge-avatar-sync when forge-token appears";
       wantedBy = [ "multi-user.target" ];
@@ -259,7 +259,7 @@ in
     };
 
     # One-shot: hyperhive.icon → Forgejo profile avatar. Shape contract:
-    # docs/conventions.md::Best-effort oneshot services.
+    # docs/process/conventions.md::Best-effort oneshot services.
     # RemainAfterExit = false so the .path trigger above can re-fire
     # this unit when the forge-token arrives after boot. The PNG is
     # rasterized at build time (`iconPng`, shared shape with the matrix