feat(marginnote4): Phase 1 — connected KB type, read-only tools, and sync bridge

[evan188199-tech]

Summary

Adds MarginNote 4 as a connected knowledge base type that syncs study data from MN4 via the official Add-on API into a dedicated SQLite store, then exposes it to the chat loop through seven read-only tools.

This follows the same architectural pattern as the Obsidian integration (#756): a KnowledgeCapability that owns the turn exclusively when an MN4 library is selected, backed by a connected KB type that skips indexing.

What's included (Phase 1)

New package deeptutor/capabilities/marginnote4/:

• models.py — MarginNoteObject, SyncBatch, PairedDevice, LearningEvent
• store.py — SQLite CRUD + search + device pairing with SHA-256 hashed tokens
• binding.py — per-turn resolution of the selected MN4 library
• tools.py — 7 read-only tools: search, read, list, documents, links, tags, cards
• capability.py — MarginNoteCapability (KnowledgeCapability, exclusive turn ownership)
• prompts/{en,zh}/system.md — bilingual system prompts

HTTP bridge deeptutor/api/routers/marginnote4.py:

Endpoint	Auth	Purpose
POST /pair	Session	Register a device, get one-time token
POST /sync	Device token	Incremental upsert + tombstones
POST /heartbeat	Device token	Liveness + object count
GET /devices	Session	List paired devices
DELETE /devices/{id}	Session	Revoke a device
GET /status	Session	Health check + stats

Integration points:

• kb_types.py: MARGINNOTE4_KB_TYPE added to CONNECTED_KB_TYPES
• manager.py: db_path surfaced in get_metadata allowlist
• registry.py: MarginNoteCapability registered in LOOP_CAPABILITIES
• builtin/__init__.py: MARGINNOTE_TOOL_TYPES in BUILTIN_TOOL_TYPES
• api/main.py: router mounted at /api/v1/marginnote4

Data sovereignty

• MarginNote 4 owns: original text, excerpts, mindmap, native cards, FSRS scheduling
• DeepTutor owns: RAG, reasoning, mastery tracking, long-term memory
• DeepTutor never reads MN4's private database or modifies its FSRS state

Test coverage

40 new tests across:

• Store: pairing, token verification, revocation, ingest (store/update/delete), search, list, documents, tags, links, cursor
• Tools: all 7 tools with seeded data, graceful failure on missing store
• Capability: binding resolution, caching, exclusivity, system blocks (en/zh), owned KBs
• KB manager: orphan prune survival, metadata surfacing, reconcile safety

All 73 tests pass (40 new + 33 existing capability/knowledge), 0 regressions. Ruff clean.

Roadmap

• Phase 2: Diff-confirmed write-back to MN4 (propose → confirm → apply → verify → rollback)
• Phase 3: Card learning event sync, mastery bridge, cross-device setup docs

Refs: #756 (Obsidian pattern), #180 (multimodal learning)

[evan188199-tech]

Rebase and validation update

• Replayed both Phase 1 commits onto the latest dev (23ad80d1).
• Added a formatter-only commit for the new MarginNote4 files.
• Re-ran validation with Python 3.11.15:
  • pytest -q tests/capabilities/marginnote4 tests/knowledge/test_marginnote4_kb.py: 44 passed
  • ruff check on changed integration files: PASS
  • ruff format --check on changed integration files: PASS
  • git diff --check: PASS

Phase 2 write-back and Phase 3 mastery bridging remain intentionally out of scope.