refactor(tasks): declare-then-execute — ctx.tasks.start() replaces register()

start({ kind, label, owner, run }) preflights everything that can fail
(the attachSurface fence, validation, the owner-cleanup attach) BEFORE
invoking the producer's run() starter, then commits atomically —
'work started but never got a collectable id' is now structurally
impossible instead of a producer try/catch rollback obligation (the
P1 review fix, rebuilt on #185's declare/execute split). Producers
lose their catch-wraps; the leak tests now pin the stronger property
that a failed preflight never spawns anything. TaskRegistration splits
into TaskStart (identity + run) and TaskHooks (cancel/done/readOutput);
docs, type-equiv manifest, catalogs, and both RFCs move with it.

packages/tasks/tool-tasks/src/index.ts

@@ -6,8 +6,8 @@
  * registry (`@deepseek-ai/dsh-tasks`).
  *
  * This plugin IS the control surface: it calls `ctx.tasks.attachSurface()` on
- * load, which is what re-arms producers' `register()` (the registry refuses
- * background work while no surface could collect or stop it).
+ * load, which is what arms producers' `ctx.tasks.start()` (the runtime's
+ * preflight refuses background work while no surface could collect or stop it).
  *
  * Completion notices: when a task settles, a short notice is injected into
  * the owning agent's session (`agent.inject()` — durable context for the NEXT