[stefan-ernst]

Why

GH #349 asks for a plain-Markdown editing mode. Decision (see the item): don't add a native source mode to Milkdown — its serializer is lossy and would become the source of truth. Instead expose a plugin extension point so an installed plugin can render its own editor (e.g. a textarea), with the host remaining the only writer.

The editor plugin itself ships from a separate repository. This PR is only the core wiring: the manifest point, validation, the extensions endpoint, the bridge protocol, and the host component.

Backend

• Extension gains mode (toggle default / replace) and entityTypes.
• LoadPlugin validates editor.markdown manifests — component required, known entity types (item/page/comment), known mode — before compilation and again for the resolved manifest+metadata set.
• GetExtensions now applies the same license gate as GetCapabilities, so an unlicensed plugin cannot advertise an editor provider.
• GET /api/plugins/extensions accepts ?point= so an editor surface only receives the point it needs.

Frontend

• Extensions store: ensureExtensionsLoaded() shares one in-flight request (the host mounts many times per page); getEditorProviders() filters by entity type and sorts by order.
• PluginBridge: rejects messages whose event.source is not the bridge's own iframe; adds the editor protocol (host:editorInit/Update/SaveRequest/CancelRequest/Dispose and plugin:editorReady/Change/Save/Cancel/Error). Every plugin → host editor message must echo the sessionId, so two editors on one page cannot cross-talk. Legacy messages still route.
• PluginEditorHost.svelte: resolves a provider, shows an opt-in toggle, mounts the plugin iframe, sends the current Markdown on ready, persists plugin:editorSave through the parent, confirms before discarding unsaved edits, refuses a save over 256 KB, and mirrors MilkdownEditor's imperative surface (insertImage/insertMarkdown no-op with a warning in plugin mode).
• ItemDetailDescription opts in for the item description.

Acceptance covered

• A manifest can declare editor.markdown and it appears in GET /api/plugins/extensions only when the plugin is enabled and licensed.
• The host mounts the provider with entity/workspace context, persists the raw Markdown via the existing API, confirms before discarding unsaved edits, and leaves the native editor unchanged when no provider exists.

Not in this PR (separate repo / follow-ups)

• The actual plaintext plugin (separate repository).
• Page-editor integration (item description is the first surface).
• insertImage/insertMarkdown delegation, mention/page-link pickers, persisted per-user preference, and the Playwright fixture plugin.

Verification

• go build ./internal/..., go vet, golangci-lint (plugins/handlers) — 0 issues
• internal/plugins/... — pass
• internal/handlers/... — pass except the two pre-existing TestNeutralInferenceEndpoint... failures (fail on main)
• bun run check, svelte-check — pass
• Full ../core-tests frontend suite — 439 files, 3283 tests pass
• New tests: core-tests#37 (Go validation/license gating; Vitest store + bridge)

Pairs with Windshiftapp/core-tests-internal#37.

[github-actions]

Size Change: 0 B 🆕

Total Size: 0 B

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