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.

apps/tui-go/internal/protocol/protocol.go

@@ -254,3 +254,9 @@ func CreateGrant(sessionID, scope string, permittedTiers []string, reason, toolN
 		"toolName":       toolName,
 	})
 }
+
+// Hello is the first frame sent on every (re)connect. It carries the client's
+// working directory so the server can bind a workspace before any session starts.
+func Hello(workingDir string) []byte {
+	return encode("Hello", map[string]any{"workingDir": workingDir})
+}