[MN4][P1] Add end-to-end coverage and integration hardening

[evan188199-tech]

Tracked by #10. Local plan: docs/roadmap/marginnote-4.md.

Goal (中文): 补齐 MN4 关键链路测试：probe/connect API、Capability 激活、Deep Research、写回 frontmatter、路径隔离和前端状态；加入 CI，并清理空的 tests/capabilities/marginnote4/ 遗留目录。

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• I believe this is a legitimate feature request, not just a question or bug.

Feature Request Description

Turn the MarginNote integration from unit-tested pieces into a regression-safe workflow.

Current coverage includes adapter/capability/KB unit tests, but there is no router-level or end-to-end path from probe/connect through capability activation, Deep Research, and writeback. There is also an empty leftover tests/capabilities/marginnote4/ directory.

Add fixtures based on real (redacted) MN4 export formats and cover:

• probe-marginnote and connect-marginnote
• capability activation and owned-KB RAG exclusion
• Deep Research read-only tool mounting
• writeback Markdown/frontmatter
• path-access isolation
• frontend connect/detail states

Include the MN4 critical path in the CI regression set.

Related Module

API/Backend

Use Case

A later parser or UI change cannot silently break connect, research, or writeback. Contributors can run one focused suite and know the MN4 workflow still works.

Additional Context

Priority: P1
Phase: M2
Status: Planned
Depends on: P0 parser, probe, detail, and Deep Research work; should track writeback and cross-link as they land
Blocks: treating v1 as merge-ready

Key files

• tests/api/test_knowledge_router.py
• tests/knowledge/test_marginnote_kb.py
• tests/capabilities/test_marginnote_capability.py
• tests/capabilities/marginnote4/ (remove leftover empty directory)
• CI workflow configs under .github/workflows

Acceptance criteria

• Tests cover probe/connect, capability activation, Deep Research mounting, writeback frontmatter, path isolation, and frontend state contracts.
• Real-format fixtures are checked in (redacted).
• Empty tests/capabilities/marginnote4/ leftover is removed.
• MN4 critical path is part of CI.
• Empty folder, corrupted fixture, denied path, and remote-inaccessible path are tested.

Success metric
The MN4 workflow has an automated regression net rather than only local unit tests.