docs: Update AGENTS.md with sfdisk issue and next steps

Updates to AGENTS.md:
- Add Issue 6: Missing sfdisk in Docker container
- Update Next Steps to reflect current build failure
- Update Docker images section with football-dev
- Update Docker containers status (none active)
- Document sfdisk root cause and proposed solution
- Update build status section with more detail

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AGENTS.md

@@ -834,12 +834,15 @@ kern.*            /var/log/kern.log
 |-------|--------|---------|
 | debian:trixie | 120MB | Base image |
 | football-test | 120MB | Test image |
+| football-dev | TBD | Development container with all tools |
 
 #### Docker Containers (Current)
 
 | Container | Status | Purpose |
 |-----------|--------|---------|
-| (debootstrap process) | 🔄 RUNNING | Bootstrapping Debian |
+| None | No active containers | Build completed/crashed |
+
+**Note**: No containers currently running. Last build attempt failed at Step 4. |
 
 ### Disk Space
 
@@ -1039,14 +1042,42 @@ docker run --rm \
 
 ---
 
+### Issue 6: Missing sfdisk in Docker Container
+
+**Problem**: `sfdisk` command not found in Docker container during disk image creation
+**Error**: `bash: line 14: sfdisk: command not found`
+**Symptoms**: Build fails at Step 4 (Disk Image Creation) after successfully completing Steps 1-3
+**Root Cause**: Minimal Debian containers (debian:trixie) don't include partitioning tools by default
+**Attempted Solution**:
+- Script tries to install `qemu-utils` and `fdisk` but sfdisk not found
+- `fdisk` package should include `sfdisk` but installation may have failed
+**Proposed Solution**:
+1. Use `football-dev` container which includes all partitioning tools (fdisk, sfdisk, parted)
+2. Alternatively, explicitly install `fdisk` package before running sfdisk
+3. Add `util-linux` package to ensure all disk utilities are available
+**Status**: ⏳ IN PROGRESS (football-dev container created, awaiting use)
+
+---
+
 ## Next Steps
 
-### Immediate (When Build Completes)
+### Immediate (Fix Build Failure)
 
-1. **Verify Output Files**:
-   - [ ] `output/football-physical.img` exists
-   - [ ] `output/football-vm.qcow2` exists
-   - [ ] Files are correct size
+1. **Fix sfdisk Missing Issue**:
+   - [ ] Use football-dev container instead of debian:trixie container
+   - [ ] Verify sfdisk is available in container: `which sfdisk`
+   - [ ] Re-run Step 4 (Disk Image Creation) with proper tools
+   - [ ] Complete disk partitioning with GPT
+   - [ ] Format filesystems (FAT32, ext4)
+   - [ ] Copy chroot to root filesystem
+   - [ ] Install GRUB for UEFI boot
+   - [ ] Convert raw image to QCOW2
+
+2. **Verify Output Files**:
+   - [x] `output/football-physical.img` exists (8GB - may be incomplete)
+   - [ ] `output/football-physical.img` is valid and complete
+   - [ ] `output/football-vm.qcow2` exists and valid
+   - [ ] Files are correct size (8GB raw, compressed QCOW2)
    - [ ] Files are readable
 
 2. **Boot VM**: