docs: migrate remaining OpenCode references to MiMoCode

[zZacharyz]

Why

Several documentation files still reference 'OpenCode' instead of 'MiMoCode'. These were carried over from the fork and never updated. Users following these docs may be confused or encounter broken links.

Files to update

• CONTRIBUTING.md - title, content, GitHub links, CLI commands
• SECURITY.md - threat model description, security report link
• packages/desktop/README.md - title and description
• sdks/vscode/README.md - all content and links
• packages/opencode/src/acp/README.md - commands, env vars, config examples
• packages/app/README.md - backend reference
• packages/opencode/test/AGENTS.md - config filename, test prefix
• packages/slack/README.md - integration description

Related PRs

• docs: rewrite opencode package README #1148: Rewrites packages/opencode/README.md (still uses old build path)
• docs: migrate OpenCode names to MiMoCode in config and CLI docs #1155: Migrates config.mdx and cli.mdx

Notes

• packages/opencode/README.md build path still references \opencode-/bin/opencode\ but actual output is \mimocode-/bin/mimo\
• Environment variable \MIMOCODE_ENABLE_QUESTION_TOOL\ should be documented consistently