feat(conductor): Wave 2 — parallel batch execution + SWITCH branching step

- Parallel batch execution: batch field on Step, batchConfig on Flow,
  batch-aware readySteps with maxConcurrent gating, getReadyInBatch helper
- SWITCH branching step: new 'switch' StepKind with cases/programmed conditions,
  resolveSwitch() pure function, switch-excluded steps tracked in
  SchedulerState, non-selected branches excluded from execution

apps/coder/src/conductor/types.ts

@@ -38,7 +38,20 @@ export interface StepContext {
   readonly model?: string;
 }
 
-export type StepKind = 'agent' | 'code' | 'approval';
+export type StepKind = 'agent' | 'code' | 'approval' | 'switch';
+
+/**
+ * One branch of a SWITCH step. The first case whose condition evaluates to true
+ * is selected; all other branches' stepIds are excluded from execution.
+ */
+export interface SwitchCase {
+  /** Human-readable label for this branch (reported in switch output). */
+  label: string;
+  /** Pure guard — called with the current step context to decide this branch. */
+  condition: (ctx: StepContext) => boolean;
+  /** stepIds belonging to this branch. */
+  stepIds: string[];
+}
 
 export type TriggerRule = 'all_success' | 'one_success' | 'all_done';
 
@@ -63,12 +76,19 @@ export interface Step {
   /**
    * For kind:'agent', returns the worker PROMPT (task + any prior outputs).
    * For kind:'code', returns the step RESULT directly (the fold/transform).
+   * For kind:'switch', unused (the runner evaluates cases internally).
    */
   run: (ctx: StepContext) => string | Promise<string>;
   /** optional guard — when it returns false the step is skipped (e.g. no repo) */
   when?: (ctx: StepContext) => boolean;
   /** max retries on timeout (0 or unset = no retry) */
   maxRetries?: number;
+  /** batch group id; steps sharing the same batch are gated by batchConfig.maxConcurrent */
+  batch?: string;
+  /** for kind:'switch' — ordered list of branches evaluated in declaration order */
+  cases?: SwitchCase[];
+  /** for kind:'switch' — fallback step ids when no case matches */
+  defaultBranch?: string[];
 }
 
 export interface Flow {
@@ -79,6 +99,8 @@ export interface Flow {
   render: (ctx: StepContext) => string;
   /** optional output filename for the artifact, derived from input */
   output?: (ctx: StepContext) => string;
+  /** batch parallelism control — gates concurrent dispatch of steps sharing the same batch id */
+  batchConfig?: { maxConcurrent: number; timeoutMs?: number; joinRule?: TriggerRule };
 }
 
 export interface RunResult {