Add --validate flag to check rules files without harmonizing

[matthewhorridge]

Summary

• harmonize --validate --rules a.json --rules b.yaml validates each rules file (JSON or YAML by extension, same as loading) and exits non-zero if any file has problems, so it can gate a CI step; --input/--output are not required in this mode (still enforced otherwise)
• New validate_rules_file in rule_registry.py: unlike RuleSet.load, which raises on the first bad rule, it checks every rule and operation and reports all problems — syntax errors, malformed sources/target/operations, unknown operations, invalid operation settings (validated by actual deserialization, so nested case/coalesce/map_each children are covered), duplicate targets within a file, and empty files
• Documented in the README under "Validating rules files"

Sample output:

bad_rules.yaml: INVALID
  rule 1 (target 'b'): operation 1 ('frobnicate'): Unknown operation: frobnicate
  rule 2: 'target' must be a non-empty string
  rule 2: operation 1 ('extract_regex'): Invalid regex pattern: '('

Test plan

• 16 new tests in tests/test_validate.py covering both syntaxes, legacy source key, every error class, multi-file exit codes, and that plain harmonization still requires --input/--output
• pytest tests — 215 passed
• Verified from a pipx install: valid file exits 0, broken file exits 1 with all problems listed

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