feat(cli): forkd from-image — Docker image → snapshot in one command

[WaylandYang]

Summary

Closes the OCI-native distribution gap. Before this PR, going from a Docker image to a forkd snapshot required two commands:
```bash
forkd parent build python:3.12-slim --output python.ext4 --extra python3-numpy
forkd snapshot --tag py-numpy --kernel ./vmlinux-6.1.141 --rootfs python.ext4 --tap forkd-tap0
```

After this PR, one command:
```bash
forkd from-image python:3.12-slim --tag py-numpy --extra python3-numpy
```

Same pipeline (Docker pull → ext4 → boot + warmup → pause → register tag), wrapped in one verb. boxlite's positioning is "docker pull X and you're done"; this matches that surface.

Design

• New `Cmd::FromImage` subcommand, dispatch in `main()`.
• New `from_image_cmd()` calls existing `parent_build_cmd` + `snapshot_cmd` internally — no new boot/snapshot machinery, just convenience chaining.
• Kernel resolution: `--kernel` arg > `FORKD_KERNEL` env > standard search paths (`./vmlinux-6.1.141`, `./vmlinux`, `/var/lib/forkd/kernels/vmlinux`, `/usr/local/share/forkd/vmlinux`). Defaults so most users won't need to pass `--kernel`.
• Rootfs cached at `/var/cache/forkd/.ext4` (same convention as `forkd run`); re-running with the same image skips the Docker step.
• All forkd-vmm boot/snapshot tuning knobs surfaced as flags (`--size-mib`, `--boot-wait-secs`, `--mem-size-mib`, `--tap`).

Smoke tests on dev box

```
$ forkd from-image --help
Build a forkd snapshot from a Docker image in one command.
...

$ forkd from-image alpine:latest --tag t --kernel /nonexistent
Error: kernel not found: /nonexistent

$ forkd from-image alpine:latest --tag t # no kernel in cwd or env
Error: no kernel found; pass --kernel or set FORKD_KERNEL. searched: ./vmlinux-6.1.141, ./vmlinux, /var/lib/forkd/kernels/vmlinux, /usr/local/share/forkd/vmlinux
```

End-to-end pipeline not run in CI (requires root + Docker + ~3 minutes per image); the underlying `parent_build_cmd` and `snapshot_cmd` are already exercised separately. This PR is convenience wiring.

Files

• `crates/forkd-cli/src/main.rs` — new `Cmd::FromImage` variant (~40 lines), dispatch (~12 lines), `from_image_cmd` + `find_default_kernel` (~95 lines). 147 total insertions, no deletions.

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