Add option to suppress non-fatal startup warnings

Summary

Please add a CLI/config option to disable or hide non-fatal yellow startup warnings in the Codex CLI.

Motivation

Some warnings are useful during setup, but can become persistent noise when they are known, non-blocking, or intentionally deferred. For example, on startup Codex currently prints warnings like:

⚠ Skipped loading 2 skill(s) due to invalid SKILL.md files.

⚠ /Users/dhruvanand/.codex/skills/canvas/SKILL.md: missing field `description`

⚠ /Users/dhruvanand/.agents/skills/canvas/SKILL.md: missing field `description`

⚠ MCP client for `html-portal` failed to start: MCP startup failed: handshaking
  with MCP server failed: connection closed: initialize response

⚠ MCP startup incomplete (failed: html-portal)

These are visible every time even when the CLI can otherwise continue normally.

Requested behavior

Add a supported way to suppress these non-fatal warnings, for example one of:

• a config setting in ~/.codex/config.toml, such as hide_startup_warnings = true
• a CLI flag, such as --quiet-startup-warnings
• a more granular setting to disable specific warning categories, such as invalid skills vs MCP startup failures

Ideally errors that prevent Codex from running would still be shown; the request is specifically about warnings that do not block startup.

Environment

• Codex CLI version: 0.133.0
• Install source: Homebrew cask
• OS: macOS