feat(workspace): Axis 2 Phase B — client→server workspace handshake

Wires the workspace handshake end to end so a session's workspace is bound
from the client's cwd at connect time and is event-sourced for replay.

- Go TUI sends Hello{workingDir=os.Getwd()} as the first WS frame on connect
  (protocol.go encoder + client.go connect path; golden test pins the wire
  format against the Kotlin discriminator).
- Server adds ClientMessage.Hello, stashes the per-connection workingDir, and
  on session start resolves it through WorkspaceResolver's trust pipeline,
  emits SessionWorkspaceBoundEvent (invariant #9), and threads the resolved
  workspace into OrchestrationConfig for the live run. A Hello after the first
  StartSession is ignored (warn); a rejected path binds the resolver fallback.
- Replay derives the workspace from the recorded event: SessionState gains
  boundWorkspace, DefaultSessionReducer fills it from SessionWorkspaceBoundEvent,
  and ReplayOrchestrator uses it (Path.of only, no filesystem re-query —
  invariant #8) with graceful fallback to config for pre-Phase-B logs.

Absent Hello / null resolver degrades to the prior config-workspace behavior.

core/sessions/src/main/kotlin/com/correx/core/sessions/BoundWorkspace.kt

@@ -0,0 +1,6 @@
+package com.correx.core.sessions
+
+data class BoundWorkspace(
+    val workspaceRoot: String,
+    val allowedPaths: List<String>,
+)

core/sessions/src/main/kotlin/com/correx/core/sessions/DefaultSessionReducer.kt

@@ -1,5 +1,6 @@
 package com.correx.core.sessions
 
+import com.correx.core.events.events.SessionWorkspaceBoundEvent
 import com.correx.core.events.events.StageCompletedEvent
 import com.correx.core.events.events.StageFailedEvent
 import com.correx.core.events.events.StoredEvent
@@ -29,10 +30,20 @@ class DefaultSessionReducer : SessionReducer {
         val createdAt = state.createdAt
             ?: event.metadata.timestamp
 
+        val boundWorkspace = when (payload) {
+            is SessionWorkspaceBoundEvent ->
+                BoundWorkspace(
+                    workspaceRoot = payload.workspaceRoot,
+                    allowedPaths = payload.allowedPaths,
+                )
+            else -> state.boundWorkspace
+        }
+
         return state.copy(
             status = newStatus,
             createdAt = createdAt,
-            updatedAt = event.metadata.timestamp
+            updatedAt = event.metadata.timestamp,
+            boundWorkspace = boundWorkspace,
         )
     }
 }

core/sessions/src/main/kotlin/com/correx/core/sessions/SessionState.kt

@@ -7,4 +7,5 @@ data class SessionState(
     val createdAt: Instant? = null,
     val updatedAt: Instant? = null,
     val invalidTransitions: Int = 0,
+    val boundWorkspace: BoundWorkspace? = null,
 )