Add cron_only frontmatter to suppress scheduled-task SKILL.md from slash autocomplete

Problem

Scheduled tasks created via the scheduled-tasks MCP land in ~/.claude/scheduled-tasks/<name>/SKILL.md. The slash-command autocomplete UI registers these alongside ~/.claude/skills/ entries, so a user typing / sees both real skills and cron-triggered tasks in the same flat list.

Concrete failure mode

A user types /journal and autocomplete surfaces TWO results:

1. /journal: the conversational journal skill (the one users actually want, 95% of the time).
2. /daily-journal: the cron-style scheduled task that auto-fires every evening to rebuild the journal index, sweep Session Captures, and invoke the skill underneath. Manual invocation defeats the purpose of having a scheduler.

The same shape hits any scheduled task that shares a stem with a real skill (graphify-weekly-check, monthly-token-optimization, etc.). New users see them, click on them, and wonder why nothing makes sense.

Current workarounds (all imperfect)

1. Naming convention: prefix scheduled tasks with _ so they sort to the bottom of autocomplete. Works visually, does not actually suppress them.
2. Convention + lint: scan ~/.claude/scheduled-tasks/ and warn on names that collide with skills. Surfaces the problem, does not fix it.

Neither makes scheduled tasks disappear from a context where they have no business showing up.

Proposed fix

Support cron_only: true in scheduled-task SKILL.md frontmatter. When set, the skill registers normally for the cron dispatcher but is excluded from the slash-command autocomplete UI.

This keeps the SKILL.md format consistent (single artifact for both directly-invokable skills and cron-triggered tasks) and gives users a clean toggle for the right behavior.

Equivalent: a separate ~/.claude/scheduled-tasks/ directory could be excluded from autocomplete by default, with an opt-in to surface specific entries. Either shape works; the frontmatter flag is the more flexible of the two.

Cross-reference

• adelaidasofia/ai-brain-starter#16: filed against ai-brain-starter from the user-experience side, with the same proposal as ask 3.
• #57447: related upstream docs gap (slash command docs omit mid-input autocomplete behavior).

Version

v2.1.136 (current). Behavior unchanged across recent versions.