Add --format json to --list-operations; hint on unknown operations in validation

[matthewhorridge]

Summary

• harmonize --list-operations --format json emits a machine-readable listing: [{operation, summary, help}] where help is the full class docstring — for case/coalesce that includes complete authoring examples, so tools and AI agents that write rules files get authoring guidance, not just names
• Validation detects unknown operation names explicitly and adds a did-you-mean suggestion (unknown operation 'normalize_bool' (did you mean 'normalize_boolean'?))
• When any unknown operation is reported, the validate output ends with hint: run 'harmonize --list-operations' to see the available operations

Sample validate output:

bad_rules.yaml: INVALID
  rule 1 (target 'b'): operation 1: unknown operation 'frobnicate' (did you mean 'truncate'?)
  rule 2: 'target' must be a non-empty string
  rule 2: operation 1 ('extract_regex'): Invalid regex pattern: '('
hint: run 'harmonize --list-operations' to see the available operations

Test plan

• New tests: JSON listing covers the full registry with non-empty summary/help and authoring examples present for case; did-you-mean suggestion; hint shown on unknown operations and absent otherwise
• pytest tests — 224 passed

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