Add automated kernel CI workflow with kselftest and PR creation

Implements a 5-stage GitHub Actions pipeline for automated kernel testing and PR creation.
Uses kernel-container-build automated-testing-v1 branch for build/test tooling.

Stage 1: Build (15-30 min)
- Checkout kernel source + kernel-container-build repo (automated-testing-v1)
- Build kernel in CIQ builder container with kABI checking
- Convert built container to QCOW2 VM image
- Upload: kernel-build.log, QCOW2 image

Stage 2: Boot Verification (2-5 min)
- Download QCOW2 image
- Boot kernel in QEMU (KVM or TCG) and validate login prompt appears
- Upload: boot logs

Stage 3: Kernel Selftests (20-40 min)
- Download QCOW2 image
- Execute comprehensive kselftests in QEMU with dual serial consoles
- Upload: kselftest TAP logs, dmesg output

Stage 4: Compare Results (1-2 min)
Purpose: Detect test regressions by comparing against base branch

Steps:
1. Checkout with full history (fetch-depth: 0) for git merge-base ops
2. Download current kselftest logs
3. Smart base branch detection:
   - For PRs: Uses PR's target branch
   - For pushes: Sorts branches by commit date, checks 30 most recent, finds closest common ancestor via git merge-base
   - Outputs: base_branch (reused by PR stage)
4. Download baseline logs from base branch (searches last 5 successful runs)
5. Compare results:
   - Counts passing/failing tests (before/after)
   - Fails if >±3 tests changed
   - Outputs: comparison_status, comparison_message

Stage 5: Create Pull Request (1-2 min)
Purpose: Auto-create/update PR after all tests pass

Prerequisites: Only runs if build + boot + kselftest passed, no regressions detected

Steps:
1. Check all stages passed and comparison_status != failed
2. Checkout (shallow: fetch-depth: 50) for commit messages
3. Download all artifacts (build/boot/test logs)
4. Extract statistics (pass/fail counts, build times)
5. Get commit info:
   - Single commit: Use commit message
   - Multiple commits: Create summary
6. Create/Update PR:
   - Reuses base_branch from compare-results (no duplication!)
   - Generate PR body with test results via create-pr-body.sh
   - Creates new PR or updates existing one

Signed-off-by: Shreeya Patel <spatel@ciq.com>

Author: shreeya-patel98