feat(memory-islands): rewrite plugin with modular mixin architecture and OWUI compatibility fixes

[skorphil]

Summary

Complete rewrite of the Memory Islands plugin following the repo's modular mixin architecture and fixing OWUI compatibility blockers from the previous incomplete attempt.

Changes

src/ — Modular mixin layout (built by build.py)

• meta.py — metadata frontmatter; requirements updated to pydantic, httpx
• _valves.py — admin Valves reduced to the four real settings (ISOLATE_BY_DEFAULT, AUTO_LEARN_MEMORIES, ENABLE_UPDATE_NOTIFICATIONS, CHECK_PREVIEW_RELEASES); removed dead PRIORITY and INHERIT_FROM_PARENT
• _database.py (new) — DatabaseMixin with all SQLite helpers, now persisted under a stable DATA_DIR (defaults to ~/.openwebui/memory-islands/) instead of the plugin directory
• _filters.py (new) — FilterMixin with inlet/outlet, payload-first folder resolution with Chats model fallback, memory learning and injection
• _commands.py (new) — CommandsMixin slash-command handler (/island-*)
• _updates.py — background update check mixin with memory-islands/ release tag prefix
• main.py — Filter entry point inheriting all mixins

OWUI compatibility fixes

• Folder resolution now reads folder_id from the filter payload first (modern OWUI), falling back to the Chats model instead of directly scraping webui.db (which broke under the new OWUI DB layout)
• DB is created under DATA_DIR (respected by OWUI deployments) rather than the read-only plugin install path
• Memory learning deduplicates facts and caps at _MAX_FACTS

Tests

• tests/test_memory_islands.py — module-level pytestmark = pytest.mark.unit; DATA_DIR pointed at tmp_path in fixtures so the real home dir is never touched; TestFolderResolution rewritten for the new async payload/Chats-fallback API; async _resolve_folder patched with AsyncMock; valve references updated to UPPER_SNAKE names
• tests/test_integration.py — valve spec/update assertions updated to the four real UPPER_SNAKE valve names

Verification

• uv run python build.py — passes (ruff check, format, unit tests, merge, integration skip)
• uv run pytest plugins/memory-islands/tests/ -m unit -v — 28 passed, 1 skipped
• uv run pytest plugins/memory-islands/tests/ -v — 28 passed, 1 skipped (integration module-skips without container)