feat: add opencode.jsonc support

[drumnistnakano]

Issue #1028: Support opencode.jsonc

Summary

Add support for opencode.jsonc format to allow configuration files with comments, following OpenCode's native support for both JSON and JSONC formats.

Changes

opencode-mcp.ts

• Modified fromFile() and fromRulesyncMcp() to detect and read both opencode.jsonc and opencode.json
• Implemented priority: opencode.jsonc preferred > opencode.json fallback
• New files default to opencode.jsonc (when no existing file)
• Changed all JSON parsing from JSON.parse() to parseJsonc() for JSONC support

opencode-mcp.test.ts

• Added 10+ comprehensive tests for JSONC file format support
• Tests cover priority ordering, global mode, local mode, content preservation
• Tests verify backward compatibility with existing opencode.json files

Design Decisions

• JSONC Preferred: Aligns with rulesync pattern (using rulesync.jsonc) and enables better documentation via comments
• Backward Compatible: Falls back to opencode.json if opencode.jsonc doesn't exist
• Existing jsonc-parser: Reused existing dependency already in rulesync (jsonc-parser@3.3.1) - no new dependencies added

Testing Results

• Type checking: ✅ pnpm run typecheck
• Code formatting: ✅ pnpm run fmt:check
• Tests: ✅ opencode-mcp.test.ts (74+ passing)

Resolves

Closes #1028