feat: add step interception points and rework execution hooks docs around @on (#6518)

* feat: add pre_step and post_step interception points on task execution

Introduces `StepContext` and the two step points in the dispatcher, and
wires them around agent execution in `task.py` (sync and async paths):
`pre_step` fires after `TaskStartedEvent` with the task context as
payload, `post_step` fires before `TaskCompletedEvent` with the
`TaskOutput`, and hook replacements are rebound in both directions.

* feat: wire pre_step and post_step on flow method execution

Dispatches the step points around each flow method with
kind="flow_method": `pre_step` receives the dumped call params and maps
returned edits back onto args/kwargs, `post_step` can rewrite the method
result before it is recorded. Conformance tests cover per-method firing
and output rewriting.

* docs: rework execution hooks page around the @on api

Replaces the standalone interception hooks catalog with a single
`execution-hooks.mdx` page that teaches `@on` as the primary way to
write hooks, covering the full ten-point catalog across task, flow, and
LLM execution. The legacy per-point decorators stay documented in a
closing section, and the `docs.json` navigation drops the removed page.

Author: lucasgomide