swarm-controller: split main.rs into jobq, routes and startup modules

no behaviour change: a pure move of items and tests into new modules, plus
visibility edits (pub(crate)) and path updates in sibling modules that named
crate-root items. same routes, same OpenAPI, 561 tests before and after.

why: #5470, new features stop landing in one 6.4k-line file by default.

files outside swarm-controller/src: docs/agent-lifecycle/agent-roster.md,
swarm-ui JobsPage.tsx and AgentsPage.tsx, nix/host-modules/swarm-controller.nix
and swarm-grafana.nix hold comments that named main.rs for items that moved.

frontend/packages/swarm-ui/src/pages/JobsPage.tsx

@@ -3,7 +3,7 @@
 // queue) pointed at swarm-controller's own GET /api/jobq/graph instead
 // of hive-c0re's — same wire shape (Vec<hive_jobq_wire::GraphNode>),
 // different endpoint, no component fork. swarm-controller's graph has
-// no real node kinds wired in yet (see swarm-controller/src/main.rs's
+// no real node kinds wired in yet (see swarm-controller/src/jobq.rs's
 // SwarmNodeKind/SwarmResourceKind), so this renders an empty tree today
 // — the page exists so the wiring is in place before the first real
 // swarm-level job (e.g. CreateAgent) lands.

frontend/packages/swarm-ui/src/pages/agents/AgentsPage.tsx

@@ -11,7 +11,7 @@
 // three separate fetches joined client-side by name; per operator
 // review feedback ("the view should be filled by a single backend
 // call") the join moved server-side instead — see
-// `swarm-controller/src/main.rs`'s `get_agents_status` handler. No more
+// `swarm-controller/src/routes/agents.rs`'s `get_agents_status` handler. No more
 // per-page joining left to do here: the wire row *is* the row this page renders.
 //
 // Owns the "+ agent" trigger too — the roster this populates is the