swarm: show each agent's effective model and effort, and where they came from

Each agent now reports the model and effort it runs on, and where each
comes from (`swarm`, `agent_nix`, `local_file` or `default`), in its
turn-state header on `$SWARM.agent-state.<agent>`: `effort`,
`model_source` and `effort_source` join `model`. The shared
`swarm_queue_client::agent_inference::{Effective, Source}` types spell
those fields for both ends.

The Bus keeps a source next to the model and effort. `Bus::new` takes it
from the local fallback chain (`harness_state::local_model` /
`local_effort`, which now return the source with the value), a `/model`
or `/effort` pick sets `local_file`, and the swarm poller sets `swarm`
or, on a clear, the local chain's source. A swarm value equal to the one
the agent already runs is now applied once, so the source moves to
`swarm` and the header is republished; before, `next()` skipped it as
unchanged.

swarm-controller subscribes to every agent's header with the reader
grant it already holds for the state relay, and keeps the latest
inference fields per agent in memory (`inference::Reported`).
`GET /api/agents/{name}/inference` now answers
`{configured, effective}`, `effective` being `null` until the agent's
first header with these fields since the controller started.
`swarmctl agent inference get` prints both; `set` reads `configured`.

Refs #5034

docs/swarm/inference.md

@@ -21,11 +21,41 @@ swarmctl agent inference clear iris
   `continue` calls may name, read on each call
   ([`subagent.md`](../tools/subagent.md#which-models-a-subagent-may-run-on)).
   The web UI's model picker doesn't use it.
-- `get` shows each field, naming the unset ones.
+- `get` shows each field, naming the unset ones, and then the model and
+  effort the agent reports running on, each with where it comes from.
 - `clear` removes every field; the agent goes back to its own values.
 
 The full flag reference is [`swarmctl-cli.md`](../tools/swarmctl-cli.md).
 
+## What an agent runs on
+
+`get` shows what the agent runs on next to what the swarm sets:
+
+```text
+model: opus
+effort: (not set; the agent's own)
+subagent_models: (not set; the agent's own)
+running model: opus (swarm)
+running effort: medium (local_file)
+```
+
+Each running value names where it comes from:
+
+| source       | meaning                                                                    |
+| ------------ | -------------------------------------------------------------------------- |
+| `swarm`      | the value set here                                                         |
+| `agent_nix`  | `services.hyperhive.agent.model` or `effortLevel` in the agent's agent.nix |
+| `local_file` | a `/model` or `/effort` pick in the agent's web UI                         |
+| `default`    | the harness's built-in default (`haiku`, `medium`)                         |
+
+When the agent starts without a swarm value, its agent.nix model wins over
+its `/model` pick, and its `/effort` pick wins over its agent.nix effort.
+
+The agent reports these whenever they change. swarm-controller keeps only
+what it has received since it started, so after a controller restart `get`
+shows `running: not reported…` for an agent until its next change (its next
+turn, or a setting changing).
+
 ## When an agent picks a change up
 
 Each agent reads its settings every 10 seconds and switches on the next
@@ -55,7 +85,7 @@ Every field is optional, and `{}` reads the same as no key.
 
 | method   | path                            | body                                                                                                                                 |
 | -------- | ------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------ |
-| `GET`    | `/api/agents/<agent>/inference` | —; answers the stored value, `{}` when none                                                                                          |
+| `GET`    | `/api/agents/<agent>/inference` | —; answers `{"configured": <the stored value, {} when none>, "effective": <what the agent reported, or null>}`                       |
 | `PUT`    | `/api/agents/<agent>/inference` | the whole value, replacing the stored one; the controller refuses unknown fields, an unknown effort, or a model name with whitespace |
 | `DELETE` | `/api/agents/<agent>/inference` | —; answers 204                                                                                                                       |
 
@@ -72,4 +102,12 @@ The subagent daemon (`hive-subagent-mcp`'s `swarm_models`) reads
 `subagent_models` with the same direct get, under the same credential, on
 each `start` or `continue` that names a model.
 
+`effective` comes from the agent's turn-state header on
+`$SWARM.agent-state.<agent>` ([`README.md`](README.md)), which carries
+`model`, `effort`, `model_source` and `effort_source`. swarm-controller
+subscribes to every agent's header with the reader grant it already holds for
+the state relay and keeps the latest per agent in memory. A header without
+those fields, from an agent that doesn't report them, leaves `effective`
+`null`.
+
 </details>