feat(cli): beautify forkd images + add forkd rmi <tag>

[WaylandYang]

Summary

Two snapshot-management polish items.

1. `forkd images` table revamped

Before:
```
TAG SIZE ROOTFS?
coding-agent-fork-prewarm-v1 2.4 GiB yes
```

After (live on dev box, 65 snapshots, 41.75 GiB total):
```
TAG SIZE MEMORY CREATED ROOTFS
branch-sb-6a0e9444-0027-1779340356 512.0 MiB 512.0 MiB 1h ago —
speculative-1779338575690 512.0 MiB 512.0 MiB 2h ago —
cli-diff-test 512.0 MiB 512.0 MiB 12h ago —
...
65 snapshots · 41.75 GiB total
```

Changes:

• New MEMORY column (memory.bin size — distinct from total which includes rootfs)
• New CREATED column (relative age: "1h ago", "3d ago", etc.)
• Dynamic TAG width
• Sort most-recent-first (was alphabetic)
• Footer with snapshot count + total bytes

2. New `forkd rmi ...` subcommand (docker-style)

```
$ forkd rmi probe-5-1779263791772987781 probe-6-1779263795516612417
✓ probe-5-1779263791772987781 (daemon)
✓ probe-6-1779263795516612417 (daemon)

$ forkd rmi nonexistent-tag
✗ nonexistent-tag (snapshot nonexistent-tag not found (no daemon entry, no disk dir at ...))
Error: 1 of 1 removals failed
```

• Tries daemon DELETE /v1/snapshots/:tag first (clean: registry entry + disk files removed atomically)
• Falls back to direct `rm -rf` on the snapshot dir when daemon is unreachable, or when daemon returns 404 (e.g., snapshot created via CLI local-boot path that never registered with the daemon)
• Reports source per tag: "(daemon)", "(disk)", or "(disk (daemon unreachable))"
• Validates the tag against the same rules the daemon enforces (1-64 chars, alnum/dash/underscore)

Files

• `crates/forkd-cli/src/hub.rs` — extended `LocalSnapshotInfo` (memory_bytes, created_at_unix); new `human_age` formatter; sort by recency
• `crates/forkd-cli/src/main.rs` — new `Cmd::Rmi` variant + `rmi_cmd` + `fallback_remove` (~70 lines); rewritten `images_cmd` (~25 lines)

Total: 167 insertions, 6 deletions.

🤖 Generated with Claude Code