docs: sync all documentation to V3 config format
- Replace config/config.example.json with V3 format (version: 3, api_keys array, channel_list)
- Update config-versioning.md: version 2→3, ConfigV2→ConfigV3, CurrentVersion=3
- Update 7 project READMEs: api_key→api_keys, add version: 3 to quick-start examples
- Update 12 security docs (ANTIGRAVITY_AUTH + credential_encryption): api_key→api_keys
- Update provider-refactoring.md: api_key→api_keys in all config examples
- Update security_configuration.md: api_key→api_keys in Before example
- Update 3 channel docs: channels→channel_list in JSON examples
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