docs(skills): add quickapps-write-integration-test skill

[andrii-novikov]

Applicable issues

• N/A — developer-tooling/documentation addition (no linked issue)

Description of changes

Adds a project skill — .claude/skills/quickapps-write-integration-test/SKILL.md — that guides creating an integration test for a QuickApps 2.0 feature (typically from a design doc under docs/designs/).

It deliberately does not duplicate .claude/rules/integration-testing.md (which documents the harness API/mechanics and auto-loads in the test dir). Instead it captures the workflow (design doc → passing test) and the non-obvious traps learned while writing the large-tool-response offload integration test:

• Caching covers only tool-deployment calls — the orchestrator (any AGENT_MODELS entry) always runs live, so a REST/internal/MCP-only flow produces no .response files to generate or commit.
• Weak cacheable models (gpt-4.1) can't drive multi-step tool flows — they fixate/loop; use a capable orchestrator and verify every model you list.
• _check_unexpected_tools — every called tool must be expected and every expected tool called ≥1; steer with a tight prompt and the smallest config_file_set.
• Tool-result attachments don't reach the final message — assert via tool-call history, not AttachmentCheck.
• Stale test server serves old routes (404) — kill/restart and curl-verify new endpoints.

Plus the self-proving marker test-design pattern and the 4-edit recipe for adding REST test tooling.

Validated TDD-style: a fresh subagent given the skill + a different design doc (dial_files_tools.md) produced a correct, pitfall-free test plan citing the skill's specific warnings.

Single new file; no code changes.

Checklist

• Title of the pull request follows Conventional Commits specification
• Documentation is updated/created (if applicable)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.