herdr phase 1a: missions runtime_kind + target_node schema

First slice of the second-runtime path. Missions now carry
runtime_kind ('zeroclaw' | 'local_herdr') + target_node_id (FK to
nodes) so the mission_orchestrator + phase executors can dispatch
differently depending on where the operator wants execution.

Migration:
  - 0055_missions_runtime_kind.sql — adds runtime_kind (NOT NULL
    DEFAULT 'zeroclaw' + CHECK), target_node_id (nullable FK ON
    DELETE SET NULL). All existing missions backfill to 'zeroclaw'
    so behavior is unchanged.
  - topology_runs also grows herdr_workspace_id / herdr_tab_id /
    herdr_pane_id text columns so a resumed run can reattach to the
    same Herdr pane instead of spawning a duplicate.

Code:
  - cm-db::repo::missions — Mission + NewMission carry the two new
    fields; all SELECTs updated; INSERT COALESCE-defaults
    runtime_kind to 'zeroclaw' when unspecified.
  - routes::missions::create — validates runtime_kind and requires
    target_node_id when kind='local_herdr' (400 otherwise).
  - lib/api/missions.ts — RuntimeKind type; Mission carries both;
    CreateMissionRequest optional fields.

Behavior is opt-in: no path exists yet to actually create a
local_herdr mission — that lands in Phase 1c (wizard picker). This
commit just makes the schema + validation in place so Phase 1b's
fleet_herdr dispatch module can key on it.

Tests: mission_orchestrator integration test still green.

crates/cm-api/src/routes/missions.rs

@@ -35,6 +35,9 @@ pub struct CreateMissionRequest {
     pub config: Value,
     #[serde(default)]
     pub phases: Vec<PhaseSpec>,
+    /// Defaults to "zeroclaw". "local_herdr" requires target_node_id.
+    pub runtime_kind: Option<String>,
+    pub target_node_id: Option<Uuid>,
 }
 
 fn default_schedule() -> Value {
@@ -120,6 +123,18 @@ pub async fn create(
     if body.title.trim().is_empty() {
         return Err(ApiError::BadRequest);
     }
+    // Validate runtime_kind + require target_node when local_herdr.
+    let runtime_kind = body.runtime_kind.as_deref().unwrap_or("zeroclaw");
+    match runtime_kind {
+        "zeroclaw" => {}
+        "local_herdr" => {
+            if body.target_node_id.is_none() {
+                return Err(ApiError::BadRequest);
+            }
+        }
+        _ => return Err(ApiError::BadRequest),
+    }
+
     let new = NewMission {
         workspace_id: user.workspace_id.as_uuid(),
         title: body.title.trim(),
@@ -130,6 +145,8 @@ pub async fn create(
         schedule: body.schedule,
         description: body.description.as_deref(),
         config: body.config,
+        runtime_kind: Some(runtime_kind),
+        target_node_id: body.target_node_id,
         phases: body
             .phases
             .into_iter()