Add modifier support to PlaceholderFormatter

Although basic interpolation is useful, we often want to handle parameters
differently, not using the standard modification we apply to them. This commit
introduces modifiers, a concept already present in Respect\Validation.

We use a familiar `{{value|modifier}}` syntax found in many popular template
engines. To handle different types of modifiers, I implemented a Chain of
Responsibility, where each modifier delegates to the next one when it cannot
handle them.

The `StringifyModifier` remains the default fallback, ensuring that we don't
lose the robust type-handling we previously built, but I've also made some
changes to it. I realise that for Respect\Validation, we have a strong need to
add "quotes" around strings because they represent values that were just
validated, but in the context of placeholder replacement, this behaviour is
undesirable.

I've created the `AutoQuoteModifier`, which quotes scalar values by default and
provides a `|raw` pipe to bypass quoting when clean output is needed. This
allows Validation to retain its quoting behaviour while keeping the default
`PlaceholderFormatter` chain simple.

Assisted-by: OpenCode (GLM-4.6)
Assisted-by: Gemini 3 (Thinking)

Author: henriquemoody

AGENTS.md

@@ -33,6 +33,27 @@ When creating new formatters:
 
 All formatters must implement the `Respect\StringFormatter\Formatter` interface.
 
+## Modifier Development
+
+When creating new modifiers:
+
+1. **Follow Chain of Responsibility pattern**: Check pipe value and delegate to next modifier
+2. **Use template structure**: Similar to `src/Modifier/QuoteModifier.php`
+3. **Test with TestingModifier**: Located in `tests/Helper/TestingModifier.php`
+4. **Handle type checking**: Always check input types before processing
+5. **Return string values**: Modifiers must return strings
+6. **Use Stringifier Quoter**: For string operations, inject `\Respect\Stringifier\Quoter` with CodeQuoter as default
+
+All modifiers must implement the `Respect\StringFormatter\Modifier` interface.
+
+## Testing Guidelines
+
+1. **Avoid PHPUnit mocks**: Create custom test implementations instead of using createMock()
+2. **Use custom test quoter**: Follow pattern in `tests/Helper/TestingQuoter.php`
+3. **Test contracts not implementations**: Verify interactions without depending on specific behavior
+4. **Make test properties public**: When using anonymous classes to access test state
+5. **Verify method calls**: Track whether methods were called and with what parameters
+
 ## Commit Guidelines
 
 Follow the detailed rules in `docs/contributing/commit-guidelines.md`: