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

pkg/channels/README.md

@@ -791,7 +791,7 @@ the top level, with channel-specific settings in the `settings` sub-key:
 
 ```json
 {
-  "channels": {
+  "channel_list": {
     "matrix": {
       "enabled": true,
       "type": "matrix",

pkg/channels/README.zh.md

@@ -790,7 +790,7 @@ channel 特定的设置放在 `settings` 子键中：
 
 ```json
 {
-  "channels": {
+  "channel_list": {
     "matrix": {
       "enabled": true,
       "type": "matrix",