feat(router): proper grounded system prompts for chat and narration

Replace the placeholder "You are a routing assistant" one-liner with two
purpose-written prompts: a conversational prompt that frames the router as
correx's operator-facing interface to the event-sourced engine (grounded in
workflow state, no fabrication, concise, steering-aware), and a dedicated
narrator prompt for buildNarrationContext that asks for one or two present-tense
sentences naming the stage and outcome.

The longer protected frame shifted token budgets, so the two L2 eviction tests
now size their budget from the measured protected-frame + per-entry cost instead
of hard-coded magic numbers, making them robust to prompt length.

core/router/src/main/kotlin/com/correx/core/router/RouterContextBuilder.kt

@@ -41,8 +41,30 @@ class DefaultRouterContextBuilder(
 ) : RouterContextBuilder {
 
     companion object {
-        private const val SYSTEM_PROMPT =
-            "You are a routing assistant. Provide guidance based on workflow state and conversation context."
+        private val SYSTEM_PROMPT =
+            """
+            You are correx's router — the conversational interface between the operator and a
+            local, event-sourced workflow engine. You are not the engine: stages, tools, and
+            approvals are run by the kernel and recorded as events. Your job is to help the
+            operator understand and steer what the engine is doing.
+
+            Ground every statement in the workflow state and conversation context you are given.
+            Never invent stages, tool results, file contents, or status you have not been shown;
+            if something is not in context, say you do not have it. Be concise and direct — plain
+            sentences, no filler. When the operator is steering an approval, help them state
+            clearly what they want changed.
+            """.trimIndent()
+
+        private val NARRATION_SYSTEM_PROMPT =
+            """
+            You are correx's router, narrating a running workflow to the operator in their live
+            feed. In one or two short sentences, in your own voice, say what just happened and
+            what comes next. Be concrete and grounded in the workflow state you are given — name
+            the stage, the outcome, and the reason for any failure or pause. Do not use lists,
+            headings, or preamble, and do not invent details you were not given. Speak in the
+            present, as events unfold.
+            """.trimIndent()
+
         private const val RECALLED_MEMORY_PREFIX = "[recalled memory]"
     }
 
@@ -288,8 +310,8 @@ class DefaultRouterContextBuilder(
     ): ContextPack {
         val systemPromptEntry = buildContextEntry(
             sourceType = "systemPrompt",
-            sourceId = "router-system",
-            content = SYSTEM_PROMPT,
+            sourceId = "router-narration-system",
+            content = NARRATION_SYSTEM_PROMPT,
             layer = ContextLayer.L0,
             role = EntryRole.SYSTEM,
         )

testing/deterministic/src/test/kotlin/RouterContextBuilderTest.kt

@@ -93,8 +93,14 @@ class RouterContextBuilderTest {
                 RouterL2Entry(StageId("s3"), summary, StageOutcomeKind.SUCCESS, Instant.parse("2026-01-03T00:00:00Z")),
             ),
         )
-        // L0 ~30 tokens; budget 145 leaves ~115. Two entries (~55 each = ~110) fit; the third (oldest) is dropped.
-        val pack = runBlocking { builderNoConv.build(state, TokenBudget(limit = 145)) }
+        // Measure the protected frame (L0) and per-entry cost, then size the budget to fit
+        // exactly two of three L2 entries — robust to system-prompt length changes.
+        val measured = runBlocking { builderNoConv.build(state, TokenBudget(limit = 10000)) }
+        val l0Cost = measured.layers[ContextLayer.L0].orEmpty().sumOf { it.tokenEstimate }
+        val entryCost = measured.layers[ContextLayer.L2].orEmpty().first().tokenEstimate
+        val pack = runBlocking {
+            builderNoConv.build(state, TokenBudget(limit = l0Cost + entryCost * 2 + entryCost / 2))
+        }
         val l2Entries = pack.layers[ContextLayer.L2] ?: emptyList()
         val retainedIds = l2Entries.map { it.sourceId }.toSet()
         // Newest two survive; oldest is dropped
@@ -218,9 +224,14 @@ class RouterContextBuilderTest {
                 RouterL2Entry(StageId("s3"), summary, StageOutcomeKind.SUCCESS, Instant.parse("2026-01-01T02:00:00Z")),
             ),
         )
-        // "Stage sN (SUCCESS): " + 100x = ~30 tokens each; L0 ~30 tokens.
-        // Budget = L0 (30) + two entries (30+30=60) = 90. Third entry doesn't fit → oldest (s1) dropped.
-        val pack = runBlocking { builderTight.build(state, TokenBudget(limit = 90)) }
+        // Size the budget from the measured protected frame + per-entry cost to fit exactly two
+        // of three entries; the third (oldest, s1) is dropped. Robust to system-prompt length.
+        val measured = runBlocking { builderTight.build(state, TokenBudget(limit = 10000)) }
+        val l0Cost = measured.layers[ContextLayer.L0].orEmpty().sumOf { it.tokenEstimate }
+        val entryCost = measured.layers[ContextLayer.L2].orEmpty().first().tokenEstimate
+        val pack = runBlocking {
+            builderTight.build(state, TokenBudget(limit = l0Cost + entryCost * 2 + entryCost / 2))
+        }
         val l2Entries = pack.layers[ContextLayer.L2] ?: emptyList()
         val retainedIds = l2Entries.map { it.sourceId }
         // Oldest entry (s1) should be dropped; s2 and s3 retained in chronological order
@@ -436,9 +447,11 @@ class RouterContextBuilderTest {
         val pack = buildPack(state, TokenBudget(limit = 10000))
         val systemEntry = pack.layers[ContextLayer.L0]?.find { it.sourceType == "systemPrompt" }
         assertNotNull(systemEntry)
-        assertEquals(
-            "You are a routing assistant. Provide guidance based on workflow state and conversation context.",
-            systemEntry!!.content,
+        val content = systemEntry!!.content
+        assertTrue(content.contains("correx"), "system prompt names the product")
+        assertTrue(
+            content.contains("workflow", ignoreCase = true),
+            "system prompt grounds the router in workflow state",
         )
     }