Agent-native parity: users can list workflows and select one for a task in
the dashboard, so agents should be able to as well. Adds two task-session
tools — fn_workflow_list (read: built-ins + user definitions) and
fn_workflow_select (assign a workflow to a task, defaulting to the current
one) — wired into the executor's customTools and exported from the engine
index. Covered by unit tests against mock stores.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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