feat(memory-islands-action): add Memory Islands Manager Action plugin (v0.2.0)

[skorphil]

Summary

Adds the Memory Islands Manager Action plugin (v0.2.0) — the companion Rich UI for the Memory Islands Filter. A message-toolbar button opens an inline panel that lists the active folder's learned facts, allows adding new facts, and deleting individual facts.

Key design points

• Shared database, read-only: reads the same $DATA_DIR/memory-islands/folder_memories.db (schema v2) via mirrored helpers (_resolve_data_dir, _init_databases, _load_folder_data) with timeout=5 and graceful sqlite3 error handling. The Action never writes the DB.
• Command-channel interactivity: add/delete round-trips are posted through the chat via parent.postMessage({type:'input:prompt:submit', data:{prompt:'<cmd>'}}, '*'); the Memory Islands Filter inlet intercepts @memory add: <fact> / @memory delete: <fact> and performs the write. No security-lowering settings required.
• Stored-XSS protection: all fact content is HTML-escaped on render; delete uses an HTML-escaped data-fact attribute recovered via btn.dataset.fact.
• OWUI compatibility: HTMLResponse is lazy-imported inside _render_panel_response (fastapi is not available in the dev environment); action() entry point is declared on the Action class for scripts/validate_plugin.py, delegating to ActionMixin.action.
• Update notifier wired with memory-islands-action/ release-tag prefix and ENABLE_UPDATE_NOTIFICATIONS / CHECK_PREVIEW_RELEASES admin valves.

Testing

• python3 build.py (plugin gate): ruff check + format + 41 unit tests + 3 integration tests — all pass.
• Integration tests verify the exact command payloads are accepted by the Filter's _parse_panel_command protocol (FR-011).
• scripts/validate_plugin.py reports OK memory-islands-action.

Notes / deviations

• Branch is based on feat/memory-islands-v020-rewrite (not trunk) because the integration test must run against the Filter's _parse_panel_command, which only exists on that branch; trunk lacks the memory-islands plugin entirely.
• The integration test file is named test_panel_command_protocol_integration.py (not test_integration.py) to avoid the pre-existing pytest basename collision between export-chat and memory-islands test modules.

Files

13 new files under plugins/memory-islands-action/ (src/, tests/, pyproject.toml, build.py, install.py, README.md, CHANGELOG.md).