Adds a scheduled GitHub Actions workflow that safely frees disk space on
self-hosted macOS runner `ci1` at 4:00 AM PST to prevent CI from running
out of space.

## Changes
- **New workflow** `.github/workflows/nightly-runner-cleanup.yml`: Runs
daily at 4 AM PST; only executes on runner `ci1`
- **Extended cleanup script** `scripts/ci-cleanup-macos.sh`:
`CI_NIGHTLY_CLEANUP=1` enables host-level cleanup:
  - `~/Library/Caches` subdirs (Homebrew, Xcode, LLVM)
  - npm cache
  - Runner `_work` dirs older than 2 days
- **Documentation** in workflows README with validation steps

## Safety
- Only allowlisted paths are deleted
- Never removes runner binaries, config, or user data
- Manual run via `workflow_dispatch` for testing

Made with [Cursor](https://cursor.com)
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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Primarily CI automation changes, but it introduces scheduled `rm -rf`
cleanup on a self-hosted runner; an allowlist/guard is present, yet
mistakes could still delete useful caches or workspaces and disrupt
builds.
> 
> **Overview**
> Adds a scheduled `Nightly Runner Cleanup` GitHub Actions workflow that
runs daily (and via `workflow_dispatch`) on self-hosted macOS runners,
guarded to execute only when `RUNNER_NAME` is `ci1`.
> 
> Extends `scripts/ci-cleanup-macos.sh` with a `CI_NIGHTLY_CLEANUP=1`
mode that additionally purges allowlisted `~/Library/Caches`
subdirectories and removes stale runner `actions-runner/_work`
directories older than 2 days, and updates the workflows README with the
new workflow and manual validation steps.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
0b108a8a7969060614311f54cd694f0454a6839b. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Adds a nightly GitHub Actions workflow to free disk on self-hosted macOS
runner ci1 at 4:00 AM PST. Extends the cleanup script with a guarded
nightly mode to remove allowlisted caches and stale work dirs so CI
doesn’t run out of space.

- **New Features**
- Added nightly-runner-cleanup.yml: runs daily at 4 AM PST; only
executes on runner ci1; supports manual workflow_dispatch.
- Updated scripts/ci-cleanup-macos.sh: CI_NIGHTLY_CLEANUP=1 cleans
Library/Caches subdirs (Homebrew, Xcode, org.llvm.clang*), npm cache,
Playwright browsers, and runner _work dirs older than 2 days; prints
mode and disk before/after.
  - Updated README with manual validation steps.

<sup>Written for commit 0b108a8a7969060614311f54cd694f0454a6839b.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

---------
Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: devin-ai-integration[bot] <158243242+devin-ai-integration[bot]@users.noreply.github.com>