feat: add Query Builder value() and pluck()

[memleakd]

Description

This PR proposes adding two small Query Builder helpers: value() and pluck().

They cover a common case where we only need one column, not a full row or result set:

$email     = $builder->where('id', 1)->value('email');
$names     = $builder->orderBy('name', 'ASC')->pluck('name');
$namesById = $builder->pluck('name', 'id');

• value(): returns the selected column from the first matching row, or null if nothing matches.
• pluck(): returns one column as an array, and can optionally use another column as the array key.

These methods behave like other terminal Query Builder methods: they use the current builder state, reset it by default, and can keep it when $reset is false. They temporarily select only the requested column or columns, so existing select() clauses are not used.

I also added Model wrappers so soft-delete behavior is preserved when calling these through a model.

Raw SQL expressions are intentionally not accepted as column names. If users need an expression, select() with get() remains the right tool.

Tests cover SQL generation, reset behavior, query failures, live results, keyed pluck results, invalid column input, and Model soft deletes.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide