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.

hive-c0re/src/forge/mod.rs

@@ -2,7 +2,7 @@
 //! config-repo mirroring, meta read-access grants. Also seeds
 //! `internal/docs` — a private repo every agent gets read-only
 //! collaborator access to for operator-curated shared content.
-//! No-op when `hive-forge` isn't running. Full design: `docs/forge.md`.
+//! No-op when `hive-forge` isn't running. Full design: `docs/integrations/forge.md`.
 
 mod ci_runner;
 pub mod config_pr_poll;

hive-c0re/src/forge/users.rs

@@ -59,7 +59,7 @@ fn config_org_avatar_png_path() -> std::path::PathBuf {
     ))
 }
 /// Per-agent token scopes (broad-but-not-admin) for tokens hive-c0re
-/// mints itself on the **internal** forge. See `docs/forge.md::Token
+/// mints itself on the **internal** forge. See `docs/integrations/forge.md::Token
 /// scopes` for the per-scope rationale. Not `pub(super)` — external
 /// forges (`dashboard/extra_forges.rs`) take an operator-pasted token
 /// verbatim, so their scope is whatever the operator's remote account
@@ -70,7 +70,7 @@ const TOKEN_SCOPES: &str = "read:user,write:user,read:notification,write:notific
 /// top of `TOKEN_SCOPES` so the host daemon can drive
 /// `/api/v1/admin/*`. Site-admin membership alone isn't enough: the
 /// token's own scope gate runs before the user-permission check.
-/// See `docs/forge.md::Token scopes`.
+/// See `docs/integrations/forge.md::Token scopes`.
 const CORE_TOKEN_SCOPES: &str = "read:admin,write:admin,read:user,write:user,read:notification,write:notification,write:repository,write:issue,write:organization,write:misc";
 
 /// Pull the access token out of forgejo's success message. Format
@@ -242,7 +242,7 @@ pub(super) async fn ensure_user_email(name: &str) {
         }
         Err(e) if is_forbidden(&e) => {
             // Core token missing admin scope — see
-            // `docs/forge.md::Token scopes` migration note.
+            // `docs/integrations/forge.md::Token scopes` migration note.
             tracing::warn!(
                 %name, %email, error = %e,
                 "forge: PATCH user email forbidden — core token likely missing admin scope. \