feat(swarm): agent_swarm — one prompt over many items, paced as a fleet

Ports kimi-code's AgentSwarm: a `prompt_template` containing `{{item}}`
expanded over an `items` list into up to 128 subagents, run to completion
and returned as one aggregate. Kigi already exceeds upstream on planning,
verification, isolation and merge via /graph; what it lacked was cheap
immediate fan-out. Entirely client-side — no new backend surface.

The engine is three pure pieces plus a runner: `plan` validates and
expands (every fault reported before a single member starts — a
half-launched swarm is expensive to unwind), `schedule` is the launch
ramp as testable arithmetic, `run` drives it against the existing
`SubagentBackend`. Reuses the single-spawn coordinator rather than
inventing a batch API.

Load-bearing decisions, each the result of a defect found in review:

- `backgrounded` is its own outcome. A member that outlives the 600s
  foreground budget is detached by the coordinator and KEEPS RUNNING;
  reporting it as failed invites the model to relaunch its item, putting
  a second agent on the same files. It is never offered for resume.
- `InFlightGuard` cancels live members on Drop. Send-now cancels the turn
  WITHOUT cancelling subagents and aborts the task; the dropped receivers
  read as "parent gone" and each child re-attaches itself. There is no
  cooperative path to use instead — `Cancellation` is constructed nowhere
  in the tree — so Drop is the only seam that fires.
- Retries and wall clock are both bounded. The swarm blocks the caller's
  turn, so every wait needs a ceiling it cannot argue past; stragglers at
  the deadline are reported as still-running, with their ids.
- `ToolKind::AgentSwarm` is its own variant: `TemplateRenderer`'s
  `by_kind` map holds one tool name per kind, so sharing `Task` would
  silently redirect `${{ tools.by_kind.task }}` in other tools' prompts.
- An explicitly requested model that cannot be validated is refused, as
  the task tool already does — one loud error beats `items.len()` quiet
  ones. Depth stays capped at 1: upstream's unlimited nesting is a
  hazard, not a feature.
- `SubagentResult.rate_limited` is classified where the typed ACP error
  code is still in hand; a scheduler re-deriving it from a formatted
  string would stop adapting the day the wording changed.
- Aggregate output is clamped per member (head+tail, loss stated):
  native tool output is truncated nowhere downstream.

42 agent_swarm tests. The fake backend awaits, so the concurrency and
ordering assertions can actually fail; the cap test also proves the
fixture can exceed the cap.

crates/codegen/kigi-tools/schema/tool_meta.schema.json

@@ -36,7 +36,7 @@
   ],
   "definitions": {
     "ToolKind": {
-      "description": "Categorizes what a tool does at a high level. Open set — consumers must tolerate unknown values (Rust deserializes them to `other` via `#[serde(other)]`). Known values: `read`, `edit`, `delete`, `list_dir`, `write`, `move`, `search`, `lsp`, `execute`, `plan`, `web_search`, `web_fetch`, `background_task_action`, `wait_tasks_action`, `kill_task_action`, `list`, `skill`, `memory_search`, `memory_get`, `task`, `enter_plan`, `exit_plan`, `ask_user`, `deploy_app`, `search_tool`, `use_tool`, `monitor`, `goal_update`, `other`.",
+      "description": "Categorizes what a tool does at a high level. Open set — consumers must tolerate unknown values (Rust deserializes them to `other` via `#[serde(other)]`). Known values: `read`, `edit`, `delete`, `list_dir`, `write`, `move`, `search`, `lsp`, `execute`, `plan`, `web_search`, `web_fetch`, `background_task_action`, `wait_tasks_action`, `kill_task_action`, `list`, `skill`, `memory_search`, `memory_get`, `task`, `agent_swarm`, `enter_plan`, `exit_plan`, `ask_user`, `deploy_app`, `search_tool`, `use_tool`, `monitor`, `goal_update`, `other`.",
       "type": "string"
     },
     "ToolNamespace": {