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/guest/ssh.go

@@ -89,6 +89,35 @@ func (c *Client) RunScript(ctx context.Context, script string, logWriter io.Writ
 	return c.runSession(ctx, "bash -se", strings.NewReader(script), logWriter)
 }
 
+// RunScriptOutput runs script on the guest and returns its stdout.
+// Stderr is discarded. Use for capturing structured output (patches, JSON,
+// file content) where mixing stderr into stdout would corrupt the result.
+func (c *Client) RunScriptOutput(ctx context.Context, script string) ([]byte, error) {
+	if c == nil || c.client == nil {
+		return nil, fmt.Errorf("ssh client is not connected")
+	}
+	session, err := c.client.NewSession()
+	if err != nil {
+		return nil, err
+	}
+	defer session.Close()
+	session.Stdin = strings.NewReader(script)
+	var stdout bytes.Buffer
+	session.Stdout = &stdout
+	// session.Stderr left nil: stderr is intentionally discarded.
+	done := make(chan error, 1)
+	go func() {
+		select {
+		case <-ctx.Done():
+			_ = c.client.Close()
+		case <-done:
+		}
+	}()
+	err = session.Run("bash -se")
+	done <- nil
+	return stdout.Bytes(), err
+}
+
 func (c *Client) UploadFile(ctx context.Context, remotePath string, mode os.FileMode, data []byte, logWriter io.Writer) error {
 	command := fmt.Sprintf("install -D -m %04o /dev/stdin %s", mode.Perm(), shellQuote(remotePath))
 	return c.runSession(ctx, command, bytes.NewReader(data), logWriter)