docs: contributing and config docs out of sync with code

Description

Two documentation pages are out of sync with the current codebase:

1. CONTRIBUTING.md still points the TUI code at packages/opencode/src/cli/cmd/tui/, but the TUI now lives in packages/tui (SolidJS + opentui). The old path no longer exists.

2. config.mdx documents project config as only opencode.json in the project root, but the code now also loads opencode.jsonc and config files placed inside .opencode/ directories (see ConfigPaths.files / ConfigPaths.directories and the .opencode directory walk in src/config/config.ts). The config lookup description in the Per project section is also outdated.

Expected

• CONTRIBUTING.md references packages/tui for the TUI code.
• config.mdx documents opencode.json / opencode.jsonc in the project root or .opencode/, and an accurate lookup walk.

OpenCode version

dev

Operating System

Linux