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-core-agent-sock/src/lib.rs

@@ -38,7 +38,7 @@ pub enum Request {
     },
     /// Pop pending messages from this agent's inbox.
     /// Delivery + ack cycle: see
-    /// `docs/conventions.md::Broker delivery + ack cycle`.
+    /// `docs/process/conventions.md::Broker delivery + ack cycle`.
     Recv {
         #[serde(default)]
         wait_seconds: Option<u64>,
@@ -55,7 +55,7 @@ pub enum Request {
     OperatorMsg { body: String },
     /// Wake-up event injected from inside the container. Recipient is
     /// implicit (this agent); `from` is caller-chosen. See
-    /// `docs/conventions.md::Wake injection` for the trust model and
+    /// `docs/process/conventions.md::Wake injection` for the trust model and
     /// typical callers. The wake is persisted in the sqlite broker
     /// like any other message — the agent can ack it via `AckUntil`.
     Wake { from: String, body: String },
@@ -68,7 +68,7 @@ pub enum Request {
     /// `query_agent_state` capability — rejected with an error otherwise;
     /// `"*"` is always rejected (use the manager socket). On the manager
     /// socket: `None` = manager self, `"*"` = hive-wide, any name =
-    /// that agent. See `docs/conventions.md::Loose-ends wire shape`.
+    /// that agent. See `docs/process/conventions.md::Loose-ends wire shape`.
     GetLooseEnds {
         #[serde(default, skip_serializing_if = "Option::is_none")]
         agent: Option<String>,
@@ -80,14 +80,14 @@ pub enum Request {
     SetStatus { text: String },
     /// Fetch identity + status for an agent. `name = None` =
     /// self-introspection; `Some(<agent>)` = target query. See
-    /// `docs/conventions.md::Agent metadata`.
+    /// `docs/process/conventions.md::Agent metadata`.
     GetAgentMeta {
         #[serde(default, skip_serializing_if = "Option::is_none")]
         name: Option<Ident>,
     },
     /// Cancel an open thread the agent owns. Authorisation +
     /// per-kind semantics in
-    /// `docs/conventions.md::Loose-ends wire shape`.
+    /// `docs/process/conventions.md::Loose-ends wire shape`.
     CancelLooseEnd { kind: CancelLooseEndKind, id: i64 },
     /// Create a git repo *through hive-c0re*. Agents can't create
     /// repos with their own forge token (`max_repo_creation = 0`); this is
@@ -98,7 +98,7 @@ pub enum Request {
     CreateRepo { repo: String },
     /// Mark every message popped since the last `AckTurn` as handled.
     /// Harness↔broker pairing fired after `TurnOutcome::Ok`. See
-    /// `docs/conventions.md::Broker delivery + ack cycle`.
+    /// `docs/process/conventions.md::Broker delivery + ack cycle`.
     AckTurn,
     /// Mark every inbox message with broker row id `<= up_to` as
     /// handled (`acked_at` set), whether still pending or already
@@ -107,12 +107,12 @@ pub enum Request {
     /// re-reading dozens of already-handled messages one turn at a
     /// time, the agent acks everything up to the id it has seen.
     /// Recipient-scoped — an agent can only ack its own rows. See
-    /// `docs/conventions.md::Broker delivery + ack cycle`.
+    /// `docs/process/conventions.md::Broker delivery + ack cycle`.
     AckUntil { up_to: i64 },
     /// Requeue every popped-but-unacked message back into the inbox.
     /// Harness fires this once at boot to recover from
     /// crashed-mid-turn sessions. See
-    /// `docs/conventions.md::Broker delivery + ack cycle`.
+    /// `docs/process/conventions.md::Broker delivery + ack cycle`.
     RequeueInflight,
     /// Harness → c0re: "I saw the `GracefulStop` signal, ran my
     /// stop-checkpoint turn (durable `/state` flushed) and am exiting my
@@ -267,7 +267,7 @@ pub enum Response {
     /// this agent. Ordered newest-first within each kind.
     LooseEnds { loose_ends: Vec<LooseEnd> },
     /// `GetAgentMeta` result. Per-field semantics + serde defaults
-    /// live in `docs/conventions.md::Agent metadata`.
+    /// live in `docs/process/conventions.md::Agent metadata`.
     AgentMeta {
         name: String,
         #[serde(default = "default_true")]