read an agent's agents.md through the swarm and edit it with git diffs

The agent's state/AGENTS.md stays the truth and its harness the only
writer. The harness mirrors the file into a new `agents-md` KV bucket,
and swarm-controller serves that copy, so a read works while the agent is
stopped. An edit is a unified diff plus the revision it was made against,
sent over the agent's existing control subject; the harness refuses a
stale base, applies the diff with `git apply --check` then `git apply` to
a temp copy, and writes the file only when both pass. Fixes #5519.

Files:
- swarm-queue-client/src/agents_md.rs, lib.rs: the bucket name and
  subject, the mirror value, the `ApplyAgentsMd` control command and its
  reply. `AgentControl` loses `Copy` because the new variant carries text.
- hive-agent/src/swarm_agents_md.rs, main.rs: the mirror task (start,
  10 s re-read, immediate republish after an edit) and the apply.
- hive-agent/src/swarm_input.rs: the control loop answers both commands.
- hive-agent/src/claude_md_watch.rs: its AGENTS.md path is shared.
- hive-agent/Cargo.toml, Cargo.lock: sha1 for the git blob id (already in
  the lock), tempfile moves from dev to runtime for the apply directory.
- swarm-controller/src/agents_md.rs, main.rs: the bucket reader and
  GET/PUT /api/agents/{name}/agents-md; test AppState literals get the
  new field.
- swarm-controller/src/agent_control.rs: the sink hands back raw reply
  bytes, so each command decodes its own reply type.
- swarm-controller-api/src/agents_md.rs: the route bodies, shared with
  swarmctl.
- swarmctl/src/agents_md.rs, main.rs: `swarmctl agent agents-md get|apply`.
- nix/host-modules/swarm-nats.nix, nix/module-eval/nats-authelia.nix: the
  agent's publish grant on its own `$KV.agents-md.<agent>` key.

git needs no new package: agent containers enable programs.git, and the
harness unit's PATH includes /run/current-system/sw.

swarm-controller/src/main.rs

@@ -55,6 +55,7 @@ mod agent_permissions;
 mod agent_renewal;
 mod agent_state_stream;
 mod agent_status;
+mod agents_md;
 mod alerts;
 mod archetypes;
 mod auth;
@@ -954,6 +955,9 @@ struct AppState {
     /// connection, several consumers" rationale as `agent_status` above,
     /// and `None` in exactly the state `status` is.
     agent_icons: Option<Arc<agent_icon::AgentIconReader>>,
+    /// Each agent's published `AGENTS.md`, on `status`'s connection like
+    /// `agent_icons`, and `None` in exactly the state `status` is.
+    agents_md: Option<Arc<agents_md::AgentsMdReader>>,
     /// What each agent last reported running on, from its turn-state
     /// headers on `status`'s connection. `None` in exactly the state
     /// `status` is.
@@ -1325,6 +1329,17 @@ fn agent_icon_reader(
     Some(reader)
 }
 
+/// The `AGENTS.md` reader, sharing the status reader's connection, and
+/// creating the bucket, as [`agent_icon_reader`] does for icons.
+fn agents_md_reader(
+    status: Option<&Arc<status::StatusReader>>,
+) -> Option<Arc<agents_md::AgentsMdReader>> {
+    let reader = Arc::new(agents_md::AgentsMdReader::new(status?.queue_client()));
+    let creating = Arc::clone(&reader);
+    tokio::spawn(async move { creating.create_when_connected().await });
+    Some(reader)
+}
+
 /// What each agent last reported running on, read on the status reader's
 /// connection — same rationale as [`wanted_writer`].
 fn reported_inference(
@@ -3612,6 +3627,7 @@ async fn main() -> Result<()> {
         wanted: wanted_writer(status.as_ref()),
         agent_status: agent_status_reader(status.as_ref()),
         agent_icons: agent_icon_reader(status.as_ref()),
+        agents_md: agents_md_reader(status.as_ref()),
         reported_inference: reported_inference(status.as_ref()),
         agent_login: agent_login(status.as_ref()),
         runs: review_classifier::runs::Runs::Live(status.as_ref().map(|s| s.queue_client())),
@@ -3630,8 +3646,7 @@ async fn main() -> Result<()> {
         alerts: alerts::from_env(),
     };
 
-    let app = build_app(state);
-    axum::serve(listener, app)
+    axum::serve(listener, build_app(state))
         .await
         .context("serving swarm-controller")
 }
@@ -3654,6 +3669,7 @@ fn build_app(state: AppState) -> axum::Router {
         .routes(routes!(get_jobq_rollup))
         .routes(routes!(get_agent_config_pr))
         .routes(routes!(get_agent_icon))
+        .routes(routes!(agents_md::get_agents_md, agents_md::put_agents_md))
         .routes(routes!(
             inference::get_agent_inference,
             inference::put_agent_inference,
@@ -3833,6 +3849,7 @@ mod tests {
             wanted: None,
             agent_status: None,
             agent_icons: None,
+            agents_md: None,
             reported_inference: None,
             agent_login: None,
             jobq: std::sync::Arc::clone(&sched),