fix(backend): drop pi --approve flag (removed in pi 0.73 @mariozechner rebrand)

pi 0.73 rpc mode is headless and runs tools without an approval gate; the
removed --approve flag made pi exit with 'Unknown option: --approve', breaking
every session spawn. Spawn args are now just --mode rpc. +regression test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/pi/pi-process-manager.ts

@@ -30,7 +30,10 @@ export class PiProcessManager {
           THT_AUTHOR: author,
           PATH: `${harnessVenvBin}:${process.env.PATH ?? ""}`,
         };
-        const child = nodeSpawn(cfg.piBin, ["--mode", "rpc", "--approve"], {
+        // pi 0.73 (the @mariozechner rebrand) removed the `--approve` flag: rpc mode is
+        // headless and runs tools without an approval gate, so passing it makes pi exit
+        // with "Unknown option: --approve". Args are intentionally just `--mode rpc`.
+        const child = nodeSpawn(cfg.piBin, ["--mode", "rpc"], {
           cwd: cfg.harnessDir,
           env,
         });