Expose current skill name as env var to hooks

Feature request

When a hook fires during skill execution, expose the currently-running skill name (e.g. jokko:create-pr) as an environment variable like CLAUDE_CURRENT_SKILL.

Motivation

Hooks that guard against direct tool use (e.g. warning when gh pr create is called outside a dedicated PR-creation skill) currently have no way to know whether the call is coming from a skill or from ad-hoc agent behavior. The workaround is to have skills prefix their commands with a marker env var (IS_CREATE_PR_SKILL=1 gh pr create ...) and grep for it in the hook — this is fragile and requires every skill to opt-in manually.

With CLAUDE_CURRENT_SKILL (or similar) available to hooks, a single PreToolUse hook could implement context-aware policies like:

• Warn on gh pr create unless running inside a *:create-pr skill
• Allow destructive git operations only from specific maintenance skills
• Log which skill triggered expensive API calls for cost attribution
• Enforce that certain tools are only called from approved skills

Proposed behavior

• CLAUDE_CURRENT_SKILL is set to the fully-qualified skill name (e.g. jokko:create-pr, ai-dev:preflight) when a hook fires during skill execution
• Empty or unset when no skill is active (ad-hoc tool use)
• Available to all hook types (PreToolUse, PostToolUse, etc.)