fix: harden event store, serialization, and grant engine; wire router chat

Event log integrity (sole source of truth):
- SqliteEventStore: serialize append/appendAll under a Mutex, enable WAL +
  busy_timeout + synchronous=NORMAL, roll back on any Throwable. Replaces the
  app-computed `SELECT MAX(seq)+1` read-modify-write that dropped events under
  concurrent appends (race was invisible to CI: only the in-memory store and
  single-threaded :memory: tests were ever exercised).
- Serialization: encodeDefaults + ignoreUnknownKeys, explicit @SerialName on all
  46 EventPayload subclasses and event-referenced enum constants, @Serializable on
  RiskLevel/RiskAction. Guards the append-only log against silent corruption from
  future class renames, field removals, or default-value changes.

Approval/grant security (Invariant: approvals cannot widen authority):
- Tier authorization is now a ceiling (request.tier.level <= max granted) instead
  of set membership; SESSION grants bind to a specific toolName instead of matching
  everything; handleCreateGrant rejects empty/over-tier/scopeless grants.

Router chat wiring (Phase 0-2): ChatInput round-trip, StartChatSession from IDLE,
router-panel layout, workflows behind Ctrl+W.

Tests: SqliteEventStore concurrency, serialization round-trip + discriminator
stability + unknown-key tolerance, adversarial grant scope/tier; updated existing
approval and reducer tests for the new grant semantics and StartChatSession effect.

testing/approvals/src/test/kotlin/GrantSemanticsTest.kt

@@ -21,10 +21,10 @@ class GrantSemanticsTest {
 
     @Test
     fun `grant permits exact tier - auto_approved`() {
-        val req = request("r1", Tier.T2)
+        val req = request("r1", Tier.T2).copy(toolName = "shell")
         val grant = ApprovalGrant(
             id = GrantId("g1"),
-            scope = GrantScope.SESSION,
+            scope = GrantScope.SESSION(toolName = "shell"),
             permittedTiers = setOf(Tier.T2),
             reason = "test",
             timestamp = Instant.parse("2026-01-01T00:00:00Z")
@@ -45,7 +45,7 @@ class GrantSemanticsTest {
         val req = request("r1", Tier.T3)
         val grant = ApprovalGrant(
             id = GrantId("g1"),
-            scope = GrantScope.SESSION,
+            scope = GrantScope.SESSION(),
             permittedTiers = setOf(Tier.T2),
             reason = "test",
             timestamp = Instant.parse("2026-01-01T00:00:00Z")
@@ -63,18 +63,18 @@ class GrantSemanticsTest {
 
     @Test
     fun `multiple grants - any match is enough`() {
-        val req = request("r1", Tier.T4)
+        val req = request("r1", Tier.T4).copy(toolName = "write_file")
         val grants = listOf(
             ApprovalGrant(
                 GrantId("g1"),
-                GrantScope.SESSION,
+                GrantScope.SESSION(toolName = "shell"),
                 setOf(Tier.T3),
                 "",
                 Instant.parse("2026-01-01T00:00:00Z")
             ),
             ApprovalGrant(
                 GrantId("g2"),
-                GrantScope.SESSION,
+                GrantScope.SESSION(toolName = "write_file"),
                 setOf(Tier.T4),
                 "",
                 Instant.parse("2026-01-01T00:00:00Z")