fix(config): refuse partial JSONC parses that wipe opencode.jsonc

jsonc-parser was returning truncated trees (often only `$schema`) when
configs contained JSON5-style unquoted keys. Config mutations then backed
up and overwrote the full file with that stub. Check parse errors on read
and refuse to overwrite unparseable files on write in web and VS Code.

Fixes #2923

Co-authored-by: serkraser <serkraser@gmail.com>

packages/vscode/src/DOCUMENTATION.md

@@ -58,6 +58,7 @@ The webview CSP permits `blob:` only for `worker-src` so shared UI parsers can r
 - `bridge-config-runtime.ts`
   - Config and skills message handlers (`api:config/*`).
   - Includes OpenCode resolution diagnostics parity handler used by shared UI (`/api/config/opencode-resolution`).
+  - OpenCode JSONC reads in `opencodeConfig.ts` fail closed on any `jsonc-parser` error or non-object result (`INVALID_JSONC`) so mutations cannot rewrite a partial `$schema`-only stub over an existing config.
 
 - `bridge-settings-runtime.ts`
   - Settings read/write and OpenCode skills discovery via API for bridge consumers.