fix(acp): ignore Cursor worktree stdout banner

[heavygee]

Summary

• Cursor agent acp --worktree prints a plain-text Using worktree: … banner on stdout before JSON-RPC.
• AcpStdioTransport treated JSON.parse failures as fatal and killed the session before cursorSessionId existed.
• Fail-soft: ignore non-JSON stdout lines (log + continue), matching the existing ignore of non-object JSON for Gemini. JSON-RPC error responses still reject pending requests.

Fixes #1085

Test plan

• bunx vitest run src/agent/backends/acp/AcpStdioTransport.test.ts (cli) — includes regression for Using worktree: …
• bun typecheck
• Dogfood: remote Cursor spawn with sessionType: worktree after merge / soup promote

[github-actions]

Findings

• [Major] Keep malformed JSON-RPC fatal outside the Cursor banner — handleLine now catches every JSON.parse failure and returns, so a corrupted/truncated JSON-RPC response is treated the same as Cursor's known banner. That can leave the matching pending request unresolved; session/prompt is sent with timeoutMs: Infinity, so a malformed prompt response can keep the remote ACP session stuck instead of surfacing a protocol failure. Evidence cli/src/agent/backends/acp/AcpStdioTransport.ts:229, context cli/src/agent/backends/acp/AcpSdkBackend.ts:452.
  Suggested fix:

          } catch (error) {
              if (this.shouldGuardAgentCli && line.startsWith('Using worktree: ')) {
                  logger.debug('[ACP] Ignoring Cursor worktree stdout banner', { line });
                  return;
              }
  
              const protocolError = new Error('Failed to parse JSON-RPC from ACP agent');
              logger.debug('[ACP] Failed to parse JSON-RPC line', { line, error });
              this.markClosed(protocolError);
              this.process.stdin.end();
              void killProcessByChildProcess(this.process);
              return;
          }

Questions

• None.

Summary

• Review mode: initial
• One major issue found: the fix needs to ignore the known Cursor worktree banner without masking malformed JSON-RPC frames. The current tests cover the banner and non-object JSON noise, but not an unknown malformed stdout line after initialization.

Testing

• git diff --check
• Not run: bunx vitest run src/agent/backends/acp/AcpStdioTransport.test.ts (bunx not found in this runner shell)

HAPI Bot

[github-actions]

[MAJOR] This now ignores every unparsable stdout line, not just Cursor's known Using worktree: banner. If the ACP agent emits a corrupted/truncated JSON-RPC response, the pending request is never rejected; for session/prompt that request has timeoutMs: Infinity, so the remote session can stay stuck instead of failing fast.

Suggested fix:

        } catch (error) {
            if (this.shouldGuardAgentCli && line.startsWith('Using worktree: ')) {
                logger.debug('[ACP] Ignoring Cursor worktree stdout banner', { line });
                return;
            }

            const protocolError = new Error('Failed to parse JSON-RPC from ACP agent');
            logger.debug('[ACP] Failed to parse JSON-RPC line', { line, error });
            this.markClosed(protocolError);
            this.process.stdin.end();
            void killProcessByChildProcess(this.process);
            return;
        }