feat: add date tool with ISO 8601 and human-readable formatting

[avoidwork]

Description

Add a date tool that returns the current date and time in ISO 8601 UTC format by default, with an optional format parameter to switch between ISO 8601 and human-readable output.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional choice)
• Performance improvement
• CI / build / tooling

Testing

Added tests/unit/tools_date.test.js with 9 tests covering:

• ISO 8601 format (default and explicit format: 'iso')
• Human-readable format (format: 'human')
• Fresh Date() instantiation (not cached)
• createDateTool factory produces correct name, description, and schema
• date tool registers in buildToolConfig with zero permissions

Also updated tests/unit/tool_index.test.js to account for the new date tool in empty-permissions assertions.

All 931 tests pass.

Coverage

• 100% line coverage maintained

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied

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