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.

swarmctl/src/main.rs

@@ -26,6 +26,7 @@
 //! for the same reason `hivectl` does not link `hive-c0re`.
 
 mod agent;
+mod agents_md;
 mod forge;
 mod inference;
 mod login;
@@ -302,6 +303,43 @@ enum AgentVerb {
         #[command(subcommand)]
         command: PermissionsVerb,
     },
+    /// Show an agent's AGENTS.md, or edit it with a diff.
+    ///
+    /// `get` shows the copy the agent last published, so it works while the
+    /// agent is stopped. `apply` hands the diff to the agent, which applies
+    /// it to the file with `git apply`; it needs the agent running.
+    AgentsMd {
+        #[command(subcommand)]
+        command: AgentsMdVerb,
+    },
+}
+
+#[derive(Subcommand)]
+enum AgentsMdVerb {
+    /// Print an agent's AGENTS.md on stdout, and its revision on stderr.
+    Get(InferenceAgentArgs),
+    /// Apply a unified diff to an agent's AGENTS.md.
+    ///
+    /// The diff names `a/AGENTS.md` and `b/AGENTS.md`, as
+    /// `diff -u a/AGENTS.md b/AGENTS.md` writes it. The agent refuses it when
+    /// the file is no longer at `--base`, or when it doesn't apply, and
+    /// leaves the file as it was.
+    Apply(AgentsMdApplyArgs),
+}
+
+#[derive(Args)]
+struct AgentsMdApplyArgs {
+    /// Agent name.
+    agent: String,
+    /// The file holding the diff.
+    diff_file: PathBuf,
+    /// The revision the diff was made against, as `get` printed it. It is
+    /// the git blob id of that text, so `git hash-object` of the unedited
+    /// file prints it too.
+    #[arg(long, value_name = "REVISION")]
+    base: String,
+    #[command(flatten)]
+    socket: ControllerSocketArg,
 }
 
 #[derive(Subcommand)]
@@ -645,6 +683,9 @@ fn main() -> Result<()> {
         Verb::Agent {
             command: AgentVerb::Permissions { command },
         } => permissions(&remote, command),
+        Verb::Agent {
+            command: AgentVerb::AgentsMd { command },
+        } => agents_md(&remote, command),
         // Same socket-resolution reasoning as `Create` above.
         Verb::Forge {
             command: ForgeVerb::MakeAdmin(args),
@@ -724,6 +765,15 @@ fn permissions(remote: &RemoteArgs, command: PermissionsVerb) -> Result<()> {
     }
 }
 
+fn agents_md(remote: &RemoteArgs, command: AgentsMdVerb) -> Result<()> {
+    match command {
+        AgentsMdVerb::Get(a) => agents_md::get(&a.socket.resolve(remote)?, &a.agent),
+        AgentsMdVerb::Apply(a) => {
+            agents_md::apply(&a.socket.resolve(remote)?, &a.agent, &a.base, &a.diff_file)
+        }
+    }
+}
+
 fn schedule(remote: &RemoteArgs, command: ScheduleVerb) -> Result<()> {
     match command {
         ScheduleVerb::Add(a) => schedule::add(