feat(move): add "vee move <vm> <target-dir>" to relocate a boot disk

[Benehiko]

Summary

Adds vee move <vm> <target-dir> — moves a VM's managed boot qcow2 disk image into another host directory (e.g. a fast NVMe) and updates the persisted config to point at the new location, without changing the global storage_path.

Only the boot disk image moves — vm.yaml, logs, control sockets, UEFI vars and cidata.iso stay under <storage_path>/<name>, matching how vee create --boot-disk-path places a new VM's disk (#38).

The command ensures the VM is shut down while its disk is moved: if it is running, vee stops it (prompting for confirmation), performs the move, then starts it again.

• -y/--yes — skip all prompts (stop + restart automatically); for scripting.
• --no-start — leave the VM stopped after the move, even if it was running.

Behaviour details

• Move uses rename on the same filesystem, falling back to copy+delete across filesystems (EXDEV).
• The persisted disk Path is set to the explicit destination file path so a custom basename (e.g. disk-os.img) survives — a bare directory would make Disk.AbsolutePath re-derive the generated disk-<vm>-<size>.<format> name and lose it.
• Rejects raw-device --boot-disk VMs (no managed image to move).
• Refuses if a file already exists at the target, so an existing disk is never overwritten.
• On a save-config failure the file is moved back, so config and disk never drift.

Changes

• internal/vm/manager.go: MoveBootDisk + findManagedBootDisk / managedBootDiskAbsPath / moveFile helpers; scratchDiskPath now reuses managedBootDiskAbsPath.
• cmd/move.go: the command with --yes / --no-start flags and directory shell completion.
• internal/vm/move_test.go: unit tests for disk selection, path resolution, and moveFile.
• docs: README command table + site/content/commands/move.md.

Testing

• go test ./internal/vm/... ./cmd/... — pass.
• golangci-lint fmt --diff && golangci-lint run — 0 issues; go build ./... — OK (pre-commit hook).
• Verified live end-to-end on a stopped VM with a real 17 GB disk and a custom disk-os.img filename: round-trip move out and back, config path correct in both directions, VM restored to its original state.

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