swarm: put the matrix registration token where the reader is granted

`swarm-bao-matrix-token` reads `secret/swarm/matrix/registration-token`
and is refused with `Code: 403 — permission denied`, measured on this
host at 16:17:49Z after a successful cert login.

path.rs makes every swarm path `swarm/<kind>/<name>/…` where Kind is a
closed set of four: agents, hives, services, controller. `matrix` sits
where a kind belongs, so policy.rs's read document — which emits exactly
`swarm/agents/*` and `swarm/hives/<hive>/*` — cannot cover it. The
module's own doc predicted this: "a misspelled kind is a 403 at provision
time rather than anything a compiler sees".

Moves the token to `swarm/hives/<hive>/matrix/registration-token`, built
through principal_prefix(Kind::Hive, …) like its per-hive sibling
queue::agent_client_path. The policy is untouched: render already grants
that prefix. mara chose this over widening the namespace.

The nix reader interpolates hyperhiveCfg.hiveName, with the no-fallback
reasoning glue-bao-tls.nix already gives at its own use of it.

path.rs's MOUNT doc justified itself by citing the old literal, which
this commit deletes; rewritten to cite the nix reader instead.

Scope: this makes the read reachable, not the value present. Nothing
writes that path yet, and a 403 says nothing about presence — the two
are separate findings and only the first is fixed here. No migration:
nothing ever wrote the old path and no read ever succeeded.

Gate: cargo fmt 0, cargo test -p swarm-secret-client 0 — 32 passed
against a 29-passed baseline with the change stashed, so the three new
tests are accounted for rather than assumed.

Refs #4308

swarm-secret-client/src/matrix.rs

@@ -26,6 +26,20 @@ pub fn account_path(agent: &str, account: &str) -> Result<String, Error> {
     Ok(format!("{prefix}/matrix/{account}"))
 }
 
+/// The path holding `hive`'s matrix registration token.
+///
+/// Keyed per **hive**, not per agent, like [`crate::queue::agent_client_path`]
+/// and unlike [`account_path`] above: one homeserver admits one hive's
+/// accounts, so the token that creates them is the hive's.
+///
+/// # Errors
+/// [`Error::PathSegment`] when `hive` contains anything but `[A-Za-z0-9_-]`,
+/// which is what keeps one hive's name from addressing another hive's secret.
+pub fn registration_token_path(hive: &str) -> Result<String, Error> {
+    let prefix = principal_prefix(Kind::Hive, hive)?;
+    Ok(format!("{prefix}/matrix/registration-token"))
+}
+
 /// What an account's path holds: the token, plus the homeserver it belongs to.
 ///
 /// The homeserver rides with the token rather than on the queue notice that
@@ -65,6 +79,32 @@ mod tests {
         assert_eq!(p, "swarm/agents/atlas/matrix/ops-relay");
     }
 
+    #[test]
+    fn the_registration_token_lands_under_the_hive_prefix_the_grant_covers() {
+        // Spelled out for the same reason as above, and with a second job here:
+        // the read policy grants `secret/data/swarm/hives/<hive>/*`, so this
+        // string is what makes the path reachable at all.
+        assert_eq!(
+            registration_token_path("pr1ma").expect("a plain name is legal"),
+            "swarm/hives/pr1ma/matrix/registration-token"
+        );
+    }
+
+    #[test]
+    fn the_registration_token_is_not_a_top_level_namespace() {
+        // The path it used to hold. `Kind` is a closed set and `matrix` is not
+        // one of its members, so a path with `matrix` as the second segment is
+        // outside every grant — which is how it came to 403 on every read.
+        let p = registration_token_path("pr1ma").expect("legal");
+        assert!(!p.starts_with("swarm/matrix/"), "{p}");
+    }
+
+    #[test]
+    fn a_traversal_in_the_hive_name_is_refused() {
+        let e = registration_token_path("../beta").expect_err("a traversal is not");
+        assert!(matches!(e, Error::PathSegment { kind: "hive", .. }), "{e}");
+    }
+
     #[test]
     fn a_segment_cannot_escape_its_own_directory() {
         // Each of these is a *different* way to address another agent's tree,

swarm-secret-client/src/path.rs

@@ -10,9 +10,9 @@ use crate::Error;
 
 /// The KV v2 mount every swarm secret lives under.
 ///
-/// A literal because the store's existing reader already hardcodes the same
-/// one (`secret/swarm/matrix/registration-token`); an option nobody sets would
-/// be two ways to say one thing.
+/// A literal because the nix-side reader spells the same one in
+/// `glue-matrix-bao-token.nix`; an option nobody sets would be two ways to say
+/// one thing.
 pub const MOUNT: &str = "secret";
 
 /// The root under [`MOUNT`] that every swarm secret lives beneath.