feat(todo): make the parallel in_progress policy configurable

Whether concurrent active tasks are legitimate depends on runtime
concurrency the tool cannot observe, but whether a deployment's agents
ever fan out is knowable at composition time. `allowParallelInProgress`
(default true) therefore replaces the hardcoded policy: the flag moves
the model-facing instruction and the accepted input together, so a
deployment running strictly sequential agents can restore the
single-active discipline from cordis.yml.

The durable-log invariant does not follow the flag. A log written while
parallel work was allowed must still replay after a deployment tightens
the policy, so the invariant stays silent on the active count.

docs/config-catalog.md

@@ -1796,6 +1796,26 @@ export interface Config {
 
 Source: [`packages/tasks/tool-tasks/src/index.ts:23`](../packages/tasks/tool-tasks/src/index.ts)
 
+## `@deepseek-ai/dsh-tool-todo`
+
+Requires: `tools`
+
+```ts config-catalog
+/** Model-facing todo tool configuration. */
+export interface Config {
+  /**
+   * Whether several todos may be `in_progress` at once (default true). True suits a deployment
+   * whose agents run work concurrently — subagents, background commands, workflow fan-out — and
+   * the description then instructs the model to mark every actively worked task. False restores
+   * the single-active discipline: the description asks for exactly one, and a call marking more
+   * is rejected.
+   */
+  allowParallelInProgress?: boolean
+}
+```
+
+Source: [`packages/todo/tool-todo/src/index.ts:29`](../packages/todo/tool-todo/src/index.ts)
+
 ## `@deepseek-ai/dsh-tool-web`
 
 Requires: `tools` · `web` · `systemPrompt`
@@ -2236,7 +2256,6 @@ These load from a `cordis.yml` entry with no `config:` block; they declare no co
 - `@deepseek-ai/dsh-tasks-local` ([`packages/tasks/tasks-local/src/index.ts`](../packages/tasks/tasks-local/src/index.ts))
 - `@deepseek-ai/dsh-timeout-policy` — requires `tools` ([`packages/timeout/timeout-policy/src/index.ts`](../packages/timeout/timeout-policy/src/index.ts))
 - `@deepseek-ai/dsh-tool-ask-user` — requires `tools` · `userInteraction` ([`packages/ui/tool-ask-user/src/index.ts`](../packages/ui/tool-ask-user/src/index.ts))
-- `@deepseek-ai/dsh-tool-todo` — requires `tools` ([`packages/todo/tool-todo/src/index.ts`](../packages/todo/tool-todo/src/index.ts))
 - `@deepseek-ai/dsh-user-interaction` ([`packages/ui/user-interaction/src/index.ts`](../packages/ui/user-interaction/src/index.ts))
 - `@deepseek-ai/dsh-workspace` — requires `storageDomain` · `sessionPersistence` ([`packages/workspace/workspace/src/index.ts`](../packages/workspace/workspace/src/index.ts))