[FEATURE] Allow skills to programmatically enter plan mode

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Skills currently cannot programmatically enter plan mode. There's no mode: "plan" frontmatter option, no way for a skill to invoke EnterPlanMode as a tool, and no mechanism for automated workflows to include a planning phase before execution.

This matters for complex multi-step skills (e.g., PR automation pipelines) where you want Claude to first produce a plan for user review, then execute. Today the only workaround is to include "enter plan mode" in the skill's prompt text, which is unreliable and doesn't integrate with the plan mode UI.

Related issues that touch adjacent aspects:

• #30438 — Plan Mode rework into a 1st-class plan system
• #30463 — Plan mode blocks execution in don't-ask mode
• #31459 — Allow hooks to control plan mode completely

Proposed Solution

Allow skills to declare a planning phase, either via:

• A mode: "plan" option in SKILL.md frontmatter (similar to the existing mode parameter on the Agent tool)
• Making EnterPlanMode / ExitPlanMode available as allowed-tools in skill frontmatter
• A skill-level directive that triggers plan mode as part of the skill's execution flow

The key requirement is that a skill author can define a workflow where Claude enters plan mode, presents a plan to the user, gets approval, and then proceeds to execution — all within a single skill invocation.