fix: Harden Task Planner acceptance criteria coverage

## Problem

Task Planner can inspect a Jira ticket and create planning artifacts while failing to cover all acceptance criteria. Because acceptance cases are currently treated as generic user requirements, some criteria can be collapsed or omitted without becoming a blocking plan-validation finding.

## Desired behavior

Task Planner should treat each acceptance criterion from Jira tickets, GitHub issues, PRDs, and user-provided work items as a first-class planning input.

## Acceptance criteria

* Each explicit acceptance criterion is extracted as a separate AC item with a stable ID, such as AC-01 or AC-02.
* The implementation plan includes an Acceptance Criteria Coverage section mapping every AC to plan steps, details references, validation tasks, and status.
* Implementation details include step-level acceptance criteria references so implementors can see what each step satisfies.
* Plan Validator flags missing or partial acceptance-criteria coverage as a blocking planning defect.
* Missing or partial coverage is High by default and Critical when it blocks stated ticket completion.
* Planning completion is blocked unless every AC is Covered or explicitly marked out of scope with rationale.