fix(core): create temp dir before saving truncated shell output

[kkhomej33-netizen]

Summary

• What changed: The shell output truncation path now ensures project runtime temp storage exists before saving the full output file.
• Why it changed: Fresh ACP sessions can encounter large shell output before any project temp storage has been initialized, which prevents the full-output file from being written and leaves oversized output untruncated.
• Reviewer focus: Confirm the fix preserves the existing behavior that truncation only replaces model-facing content when the full output is successfully saved.

Validation

• Commands run:
  npx vitest run src/utils/truncation.test.ts from packages/core
  npm run build
  npm run typecheck
• Prompts / inputs used: Unit test coverage exercises oversized output that needs to be saved before truncation can report a full-output path.
• Expected result: Oversized output creates project temp storage before saving the complete output file, then returns truncated output with the saved file path.
• Observed result: The focused truncation test passed, and repository build and typecheck completed successfully.
• Quickest reviewer verification path: Run npx vitest run src/utils/truncation.test.ts from packages/core.
• Evidence: Local command output reported 10 passing tests for src/utils/truncation.test.ts. Build and typecheck exited successfully.

Scope / Risk

• Main risk or tradeoff: Low. The change only creates the already-selected project temp directory immediately before writing the full saved output.
• Not covered / not validated: End-to-end ACP reproduction was not run locally.
• Breaking changes / migration notes: None.

Testing Matrix

	macOS	Windows	Linux
npm run	tested	not tested	not tested
npx	tested	not tested	not tested
Docker	not tested	not tested	not tested
Podman	not tested	N/A	N/A
Seatbelt	not tested	N/A	N/A

Testing matrix notes:

• Validation was local on macOS only.

Linked Issues / Bugs

Fixes #3874

[wenshao]

No issues found. LGTM! ✅ — gpt-5.5 via Qwen Code /review

[tanzhenxin]

Review

Clean one-line fix for #3874: mkdir { recursive: true } before writing the saved-output file, so fresh ACP sessions that hit oversized shell output before any project temp storage exists no longer end up with an untruncated payload. Tests cover both the below-threshold (mkdir not called) and truncating (mkdir called) paths, and the existing try/catch already absorbs mkdir failures via the error field — no new failure mode introduced.

Verdict

APPROVE — minimal, targeted, with appropriate test coverage.