Activate a named skill so its instructions load into the current task.
## What it does
A single call activates ONE skill, chosen by the `skill` parameter from the
roster of skills indexed for this workspace; its instructions are in your
context from your next step. A roster line may be shortened to save context —
the skill behind it is complete, and activating it loads everything.
## When to use
- The task matches an available skill — activate it BEFORE producing other output
about the task, so its guidance is in force while you work.
- The user references a skill by name or types `/<name>`.
## When NOT to use
- The skill you want is not in the roster — never guess or invent a name; pick
only from the listed ones.
- A skill is already active — do not re-activate it.
- No listed skill covers the task — just proceed without one.
