feat: add configurable regex_match evaluator

[nawaaaaaAaar]

Summary

Closes #1. Adds the requested regex_match evaluator with configurable search/full-match behavior and case sensitivity.

• Exported and registered through the existing evaluator plugin system.
• Returns failed evaluation results for invalid patterns and agent errors rather than raising.
• Includes pattern, flags and matched text in structured details.
• Does not change core models, runner orchestration or existing evaluators.

Verification

• Before implementation, the new tests failed to import the missing evaluator.
• make check: Ruff, strict mypy and the complete suite passed; 49 tests passed, with 90% reported overall coverage.
• Ten new evaluator cases cover matches, misses, case behavior, full matching, empty matching, invalid syntax, agent-error precedence and registry instantiation.
• Focused branch coverage: 100% for the new evaluator.
• uv run harness list-plugins evaluators lists regex_match.
• git diff --check: passed.

Python 3.13; no model APIs or live agents were called. Like standard-library re, this evaluator does not impose a match timeout; patterns should be trusted benchmark configuration rather than hostile input.

Prepared with Perplexity Computer assistance; no personal account-owner review is claimed.

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