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/web/server/lib/opencode/DOCUMENTATION.md

@@ -71,7 +71,8 @@ This module provides OpenCode server integration utilities for the web server ru
 - `ensureDirs()`: Creates required OpenCode directories.
 - `parseMdFile(filePath)`, `writeMdFile(filePath, frontmatter, body)`: Markdown file operations with YAML frontmatter.
 - `getConfigPaths(workingDirectory)`, `readConfigLayers(workingDirectory)`, `readConfig(workingDirectory)`: Config file operations with layer merging (user, project, custom).
-- `writeConfig(config, filePath)`: Writes config with automatic backup.
+- `readConfigFile(filePath)`: Reads one config file. Empty/missing files return `{}`. Any `jsonc-parser` error or non-object result throws `INVALID_JSONC` — partial parse trees must never be treated as authoritative (avoids rewriting a `$schema`-only stub over a full config).
+- `writeConfig(config, filePath)`: Writes config with automatic backup. Refuses to overwrite an existing non-empty file that fails the same JSONC parse check.
 - `getJsonEntrySource(layers, sectionKey, entryName)`: Resolves which config layer provides an entry.
 - `getJsonWriteTarget(layers, preferredScope)`: Determines write target for config updates.
 - `getAncestors(startDir, stopDir)`, `findWorktreeRoot(startDir)`: Git worktree helpers.