fix: promote post-execution hook dispatch to H2 with directive language (#2713)

* fix: promote post-execution hook dispatch to H2 with directive language

Restructure the after_* hook dispatch in all five core command templates
(specify, clarify, implement, plan, tasks) to make hook execution
structurally unmissable by LLMs during interactive slash-command flows.

Changes applied consistently across all five templates:

1. Promote hook block from a final numbered step to a top-level
   '## Mandatory Post-Execution Hooks' H2, placed before the completion
   report. An H2 boundary is salient in a way that a numbered sub-step
   buried after 'Report completion' is not.

2. Use directive language for mandatory hooks: 'You MUST emit
   EXECUTE_COMMAND: for each mandatory hook' and 'You MUST complete this
   section before reporting completion to the user.' The previous
   conditional framing ('check if', 'based on its optional flag') buried
   the mandatory cases.

3. List mandatory hooks before optional hooks in the dispatch block, so
   the required action appears first.

4. Add a terminal '## Done When' verification checklist at the end of
   each template, including 'Extension hooks dispatched' as an explicit
   completion criterion. This gives the model a structured opportunity to
   verify completion before exiting.

5. Extract the completion report into its own '## Completion Report' H2
   section, clearly separated from the hook dispatch.

These changes preserve the interactive-vs-workflow distinction and do not
introduce auto-run. They raise the reliability of the existing
best-effort dispatch mechanism.

Fixes #2688

* fix: address review — narrow Done When wording and move to end of templates

- Narrow the hooks checklist item from a broad condition to 'dispatched
  or skipped according to the rules in Mandatory Post-Execution Hooks
  above' so it does not contradict the filtering rules for disabled
  hooks or hooks with non-empty conditions.

- Move the Done When section to the actual end of specify.md, plan.md,
  and tasks.md so it does not signal premature completion before the
  agent reads required guidance sections (Quick Guidelines, Phases/Key
  rules, Task Generation Rules).

* fix: update stale step 8 reference in specify.md validation flow

The old 'proceed to step 8' pointed to the completion report step that
was renumbered when hook dispatch was promoted to its own H2 section.
Update the reference to point to 'Mandatory Post-Execution Hooks'.