[N12] todo.updated stream event and useTodos() for React, Vue and Svelte

[LinuxDevil]

Every successful todo_write now emits a typed todo.updated event, the shared UI reducer keeps the list, and useTodos() (React, Vue) / loushoTodos() (Svelte) expose it with counts, the current item and progress. isAgentEvent() is exhaustive, which also fixes remote clients dropping agent.drift.

Closes #254

Changes

• todo_write brands its result (non-enumerable Symbol.for('lousho.todoList')); toolSettled emits todo.updated { todos, counts, toolCallId } right after tool.done (sub-agents too, with subagent). todo_read is not branded.
• EVENT_TYPE_MAP: Record<AgentEventType, true> replaces the hand-written list (adds agent.drift and todo.updated; schema version stays 1).
• AgentUIState.todos (kept across turns, cleared by ui.reset, sub-agent updates ignored); todoView() in src/ui/todos.ts.
• useTodos(agent) (React, memoized), useTodos(agent) (Vue, one ComputedRef per field), loushoTodos(agent) (Svelte); useLoushoAgent / loushoAgent expose todos.
• toUIMessageStream() sends a data-lousho-todos part { todos, counts } (id todos).
• Agent Forge: no exhaustive switch over event.type exists, nothing to change; its four checks pass.

For the docs site

New headings (appended at the end of the page, so existing heading positions are unchanged): ## Todos in docs/react.md, docs/vue.md and docs/svelte.md. No new page.
Edited without new headings: docs/stream-events.md (new todo.updated row in the event table), docs/tools.md ("Todo tools": one added sentence), docs/ai-sdk-ui.md (new table row), docs/react.md (todos row in "Returned state"), docs/vue.md and docs/svelte.md (todos added to the state lists).

Verification (after merging origin/main, 814add8)

• npx tsc --noEmit: clean. npm run lint: zero warnings. npm run build and the create-lousho-agent build: ok.
• npm run test:types: 13 files passed, no type errors. docs:verify-snippets -- --skip-build: all 210 snippets type-check, 8 run. docs:llms:check: ok.
• npm run test:coverage: 230 files passed (one run hit a load-related timeout in http.test.ts "rejects near the configured timeout"; it passes alone and on the re-run). npm run fallow: 0 above threshold, exit 0.
• Agent Forge: typecheck, typecheck:server clean; test 112 passed; test:server 122 passed.
• npm run pack-smoke: all checks passed.
• typecheck-ai7 steps (ai@7, tsc, full vitest) and typecheck-zod4 steps (zod@4, tsc, test:types, full vitest): passed locally before the merge; default dependencies restored with npm ci, no lockfile change.

Live test spend: none (no live calls).

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