fix(wsh): accept block reference as positional arg for deleteblock (#3430)

## Summary

The `wsh deleteblock` command previously only accepted a block reference
via the `--block/-b` flag, with no Args validation, causing user
confusion when positional arguments were silently ignored and the
current block was deleted instead.

## Changes

- Accept an optional positional block-ref argument (e.g. `wsh
deleteblock 2`)
- Add `Args: cobra.MaximumNArgs(1)` to validate argument count
- Add Long description documenting accepted reference formats
(block:oid, uuid, block number, keywords like 'this')
- Add `resolveBlockArgWithOverride()` helper that prefers positional arg
over `--block/-b` flag, defaulting to 'this' when neither is specified
- Update Use string to show the optional `[block-ref]` argument

## Usage

```bash
# Delete current block (default)
wsh deleteblock

# Delete block by number (1-indexed in current tab)
wsh deleteblock 2

# Delete block by full ORef
wsh deleteblock block:abc123-def456

# Delete block using -b flag (still supported)
wsh deleteblock -b 2
```

Fixes #3417