fix(harness): streamline system prompt handling and enhance message selection logic

console/web/README.md

@@ -89,7 +89,6 @@ src/
     backend/             # ← the seam. ChatBackend interface + impls
       types.ts           #     StreamEvent, ChatBackend, ChatStreamOptions
       harness-send.ts    #     harness::send / stop / status wire helpers
-      system-prompt.ts   #     per-mode + identity system prompt
       turn-events-live.ts     # harness::turn-completed subscription
       approval-events-live.ts # approval::pending-* subscription + list-pending
       translate.ts       #     trigger payloads → StreamEvent

console/web/src/lib/backend/harness-send.ts

@@ -37,9 +37,13 @@ export type HarnessOutputContract =
   | { type: 'text' }
   | { type: 'json'; schema?: unknown }
 
+/** Operating mode — the harness prepends a short paragraph before the identity prompt. */
+export type HarnessSendMode = 'plan' | 'ask' | 'agent'
+
 /** Per-send options frozen onto the turn record. */
 export interface HarnessSendOptions {
   system_prompt?: string
+  mode?: HarnessSendMode
   max_turns?: number
   thinking_level?: HarnessThinkingLevel
   output?: HarnessOutputContract

console/web/src/lib/backend/real.ts

@@ -1,7 +1,9 @@
 /**
  * iii-browser-sdk + harness turn kickoff. Deployment permission rules live in
  * the `approval-gate` configuration entry; the console derives the harness
- * structural floor from those rules on each send.
+ * structural floor from those rules on each send. Operating mode is passed to
+ * the harness, which assembles the provider-specific identity prompt when
+ * `system_prompt` is omitted.
  *
  * Transcript content (tokens, message snapshots, function-call cards, results)
  * renders from session-manager events reconciled by the conversations layer
@@ -30,7 +32,6 @@ import {
   sendTurn,
   stopTurn,
 } from './harness-send'
-import { buildModeSystemPrompt } from './system-prompt'
 import {
   isTerminalSource,
   type TurnSourceEvent,
@@ -176,7 +177,7 @@ async function* realStream(
       idempotency_key: messageId,
       session: { metadata: { surface: 'console' } },
       options: {
-        system_prompt: buildModeSystemPrompt(mode, provider),
+        mode,
         functions: functionPolicy,
         ...(thinkingLevel ? { thinking_level: thinkingLevel } : {}),
         metadata: { session_id: sessionId, message_id: messageId },