Allow disabling built-in skills (e.g. `schedule`) via settings

Problem

Built-in skills are injected into the system prompt of every Claude Code session. The schedule skill (which exposes ScheduleWakeup, CronCreate, CronList, CronDelete, plus the /schedule slash-command framing) costs roughly 400 tokens per session.

For users or organizations that have cloud agents disabled via managed settings, /schedule and the associated tools cannot be invoked, but the skill text is still present in the system prompt. There is currently no way to opt out of it.

Proposed solution

A settings key — honored in both user settings.json and managed-settings.json — to disable named built-in skills:

{
  "disabledBuiltinSkills": ["schedule"]
}

Behavior:

• Disabled skills' system-prompt sections are omitted.
• Their slash commands (/schedule) and tools (ScheduleWakeup, CronCreate, …) are unregistered, so the model can't try to call them.

Relationship to existing issues

• #52802 proposes overriding named directive sections of the core system prompt (tone, output formatting, plan-mode framing). Its author explicitly scopes out skill injection, so it does not cover this case.
• PR #46024 (--exclude-dynamic-system-prompt-sections) is print-mode only and does not target skills.