Document the plan, revise, approve, and implement workflow in mecatui #2388

Reader need

"I want Mecatl to investigate and propose a change, let me revise the plan, and implement only after I approve it."

Current gap

Plan semantics live in user-docs/features/security-and-execution/permissions-and-posture.md; client controls and model configuration live elsewhere. Readers need an end-to-end task showing how these pieces fit together.

Proposed work

Add a focused workflow under Use Mecatl, extending an appropriate existing page if practical. Use one realistic repository change to show entering plan mode, requesting a plan, giving revision feedback, approving through the plan review, and inspecting the implementation result. Link to canonical permission and routing behavior.

Acceptance criteria

• The scenario includes a copyable task prompt and identifies the actual client mode/review controls needed.
• Readers can revise or decline a plan, approve implementation, monitor progress, and inspect the resulting diff and test evidence.
• Prose assent is distinguished from the actual plan approval flow; mode restrictions and cancellation consequences are linked to their canonical owner.
• Local and remote availability is checked against shipped behavior, including the unresolved remote approval report in Plan mode can't move on to implementation in mecak8s #1547. Do not imply that documentation resolves that product issue.
• The guide is reachable from Use Mecatl and relevant TUI guidance, and links to model routing when readers want a separate planning model.

Tracking

Part of #2386. Follow the parent issue's shared ownership and validation criteria.