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.

docs/turn-loop/README.md

@@ -14,7 +14,7 @@ agents) runs:
    queued and unacked, so a resume drains the backlog instead of
    losing it; reminders and todo wakes buffer in their channels. Set
    it with `hivectl agent <name> pause` or the dashboard toggle; see
-   [persistence](../persistence.md#-harnesspaused-per-agent).
+   [persistence](../agent-lifecycle/persistence.md#-harnesspaused-per-agent).
 1. Long-poll `Recv` on its socket. The host-side broker
    (`broker.rs::recv_blocking_batch`) returns immediately if there's
    a pending message, otherwise waits up to 30 s for a broker `Sent`
@@ -91,21 +91,21 @@ doc for the trait shape.
 `serve_main` reads `HIVE_PORT` (default `DEFAULT_WEB_PORT`) +
 `HIVE_LABEL` (default `"hive"` for standalone runs; the meta
 flake sets it unconditionally for any container-deployed agent;
-see `docs/conventions.md::Hive identity` for the env stack),
+see `docs/process/conventions.md::Hive identity` for the env stack),
 opens turn-stats sqlite, prepares the on-boot files (see
 [claude-invocation](claude-invocation.md#on-boot-files)),
 installs claude plugins, spawns `web_ui::serve` + `vacuum::run`,
 and either drops into `serve_loop` directly (`Online`) or parks on
 the login flow first (`NeedsLogin`). Forge notifications are polled by
 their own process, not this loop — see `hive-forge-notify` in
-[`forge.md`](../forge.md).
+[`forge.md`](../integrations/forge.md).
 
 Boot also opens the todos store and the socket in-container producers
 dial. Matrix / bash / forge-notify daemons and the in-process
 `disk_watch` todo producer (low state-disk space) are the built-in
 producers, but the socket accepts any `subsystem` marker — a
 user-configured MCP server can push its own todos the same way. See
-[`docs/persistence.md`](../persistence.md#state-dirs-per-agent) for
+[`docs/agent-lifecycle/persistence.md`](../agent-lifecycle/persistence.md#state-dirs-per-agent) for
 what each built-in todo producer watches and how the store + `get_loose_ends`
 merge work.