Add CODEX_BIN override for custom Codex CLI path

[gugglegum]

This PR adds an optional CODEX_BIN environment variable that lets TeleCodex use a custom Codex CLI executable instead of the SDK default.

This is useful for local or patched Codex CLI builds, while preserving the existing behavior when CODEX_BIN is unset.

Changes included:

• Parse CODEX_BIN through TeleCodex config
• Pass the configured path to @openai/codex-sdk via codexPathOverride
• Document the new option in README.md
• Add CODEX_BIN to .env.example
• Add tests for config parsing and SDK option forwarding

Validation:

• npm run build
• npm test