fix(#2860): no loopback default for the matrix homeserver

Third and last of #2860's agent-facing URL fallbacks. The operator's
ruling was "any special casing is done on the nix side - same binaries,
no hard coded fallback", so the default is deleted rather than replaced.

Every layer guessed the same wrong thing, and each guess was only ever
correct for a process sharing the host netns:

- nix/agent-modules/matrix.nix: matrixUrlDefault = localhost:8008, both
  as the option's default and as a sentinel the daemon unit compared
  against to decide whether to write HIVE_MATRIX_URL. Now nullOr str,
  default null, the guard is != null, and the doc says what forge.url's
  already says: null means "no matrix", not "guess one".
- nix/host-modules/hive-c0re/environment.nix: forwarded
  http://127.0.0.1:<port> when no gatewayHost was set. hive-c0re shares
  the host netns so it reads as harmless, but the value is handed to
  agents, which do not -- there it names the agent itself. Now forwarded
  only when there is a gateway vhost to name, matching the guard
  HIVE_MATRIX_PUBLIC_URL already uses twelve lines below.
- hive-matrix-mcp: paths::DEFAULT_HOMESERVER was the same address
  compiled in, so dropping the nix defaults alone would have left the
  daemon dialling loopback inside the agent's own netns -- the very bug,
  one layer down. homeserver_url() is now Option, and an account with no
  homeserver is skipped with a log, exactly as one with no token is.
  discover_token_accounts already refused to guess for the same reason.

Two comments taught the assumption back to the next reader ("shared host
netns means every agent container resolves localhost to the same
machine"); both now say which side of the netns boundary they describe.
MATRIX_HTTP keeps its value -- hive-c0re really does share the host
netns -- but no longer claims agents do.

Gated with nix eval against the extended agent-base config, as a pair:
with no url set the daemon unit carries no HIVE_MATRIX_URL, and with one
set it carries exactly that. Either check alone passes on a broken guard.

hive-c0re/src/matrix.rs

@@ -18,9 +18,14 @@ use crate::coordinator::Coordinator;
 /// `hive-forge` and matches the bare-name allow-list the lifecycle
 /// scanner skips over.
 const MATRIX_CONTAINER: &str = "hive-matrix";
-/// Local-host URL of the tuwunel client-server API. Shares the host
-/// netns so `localhost:<port>` resolves both from the daemon and from
-/// inside any sub-agent container.
+/// Local-host URL of the tuwunel client-server API, for **this daemon
+/// only**: hive-c0re and the homeserver share the host netns, so
+/// `localhost:<port>` reaches it from here.
+///
+/// Deliberately not an agent-facing address. An agent has its own netns,
+/// where `localhost` is the agent — agents are handed the gateway vhost
+/// (`matrix.<domain>`) via `HIVE_MATRIX_URL` instead, and get nothing at
+/// all when the hive has no vhost to offer.
 const MATRIX_HTTP: &str = "http://localhost:8008";
 /// Length (bytes) of the random registration token. 32 raw bytes ⇒
 /// 64-char hex string; comfortable for a long-lived shared secret.