Add guest.session.send and vm.workspace.export RPCs

guest.session.send — write to a pipe-mode session's stdin without
holding the exclusive attach. The daemon dials a fresh SSH connection,
uploads the payload to a temp file, and cats it into the session's
named FIFO. Linux atomicity for writes ≤ PIPE_BUF covers all pi RPC
JSONL lines. Attach exclusivity is unchanged.

vm.workspace.export — pull changes from guest back to host. Runs
`git add -A && git diff --cached HEAD --binary` inside the guest via a
new RunScriptOutput helper on guest.Client (stdout-only capture,
distinct from RunScript which merges stderr). Returns a binary-safe
patch and a list of changed files. CLI writes the patch to stdout for
`| git apply` or to a file via --output.

RunScriptOutput is implemented as a direct SSH session (same pattern as
runSession) rather than going through StartCommand/StreamSession to
avoid closing the underlying Client, which is required since
ExportVMWorkspace calls it twice on the same connection.

New files: internal/daemon/workspace_test.go

internal/api/types.go

@@ -203,6 +203,29 @@ type GuestSessionAttachBeginResult struct {
 	StreamFormat    string             `json:"stream_format"`
 }
 
+type GuestSessionSendParams struct {
+	VMIDOrName      string `json:"vm_id_or_name"`
+	SessionIDOrName string `json:"session_id_or_name"`
+	Payload         []byte `json:"payload"`
+}
+
+type GuestSessionSendResult struct {
+	Session      model.GuestSession `json:"session"`
+	BytesWritten int                `json:"bytes_written"`
+}
+
+type WorkspaceExportParams struct {
+	IDOrName  string `json:"id_or_name"`
+	GuestPath string `json:"guest_path,omitempty"`
+}
+
+type WorkspaceExportResult struct {
+	GuestPath    string   `json:"guest_path"`
+	Patch        []byte   `json:"patch"`
+	ChangedFiles []string `json:"changed_files"`
+	HasChanges   bool     `json:"has_changes"`
+}
+
 type VMWorkspacePrepareParams struct {
 	IDOrName   string `json:"id_or_name"`
 	SourcePath string `json:"source_path"`