Feature: Allow skills with context: fork to run in background

Problem

When a skill uses context: fork, it spawns a subagent that blocks the main conversation until completion. For skills that perform autonomous workflows (e.g., creating PRs, running CI checks, monitoring deployments), there's no reason to block the user from continuing their work.

Proposed Solution

Add a background: true option for skills with context: fork, similar to how async: true works for command hooks. The skill would run in the background and notify the user when complete.

---
name: pr-to-staging
context: fork
background: true
---

Use Case

I built a /pr-to-staging skill that:

1. Analyzes git diff (SessionStart hook injects commits + stat)
2. Generates PR title and body
3. Pushes and creates/edits PR
4. Monitors CI status with gh pr checks --watch

This entire workflow is autonomous — no user interaction needed. Currently it blocks the main session for several minutes while CI runs. Running in background would let the user continue working and get notified when the PR is ready with CI results.

Current Workaround

None. The async: true option exists for hooks but not for skills. The skill must block the main conversation even though the user has no input to provide.