fix(docs): revert wrong schema URL canonicalization (file is oh-my-opencode.schema.json)

In #3859 I bulk-replaced `oh-my-opencode.schema.json` ->
`oh-my-openagent.schema.json` across docs/, on the false assumption
that the schema file had been renamed alongside the package.

It hadn't. Repo state on dev:

- assets/oh-my-opencode.schema.json (only file present)
- package.json export `./schema.json` -> `./dist/oh-my-opencode.schema.json`
- script/build-schema.ts emits the same legacy basename
- root AGENTS.md line 135 documents the same legacy URL

So every doc that pointed at `oh-my-openagent.schema.json` was
returning HTTP 404 for any user copying the example into their config.

Verified live:

  https://raw.githubusercontent.com/code-yeongyu/oh-my-openagent/dev/assets/oh-my-openagent.schema.json -> 404
  https://raw.githubusercontent.com/code-yeongyu/oh-my-openagent/dev/assets/oh-my-opencode.schema.json -> 200

Files reverted to canonical legacy URL:

- docs/examples/{coding,default,planning}-focused.jsonc
- docs/guide/{overview,installation,agent-model-matching}.md
- docs/reference/configuration.md (2 occurrences)

docs/reference/configuration.md

@@ -62,7 +62,7 @@ Enable schema autocomplete:
 
 ```json
 {
-  "$schema": "https://raw.githubusercontent.com/code-yeongyu/oh-my-openagent/dev/assets/oh-my-openagent.schema.json"
+  "$schema": "https://raw.githubusercontent.com/code-yeongyu/oh-my-openagent/dev/assets/oh-my-opencode.schema.json"
 }
 ```
 
@@ -74,7 +74,7 @@ Here's a practical starting configuration:
 
 ```jsonc
 {
-  "$schema": "https://raw.githubusercontent.com/code-yeongyu/oh-my-openagent/dev/assets/oh-my-openagent.schema.json",
+  "$schema": "https://raw.githubusercontent.com/code-yeongyu/oh-my-openagent/dev/assets/oh-my-opencode.schema.json",
 
   "agents": {
     // Main orchestrator: Claude Opus or Kimi K2.5 work best