feat: add Codex CLI backend support

[CCALITA]

Summary

• Add OpenAI Codex CLI as a new backend option (backend = "codex" in config)
• New CodexBackend in internal/backend/codex.go that invokes codex -q --approval-mode suggest <prompt> for non-interactive streaming output
• Add codex_path config field and INLINE_CLI_CODEX_PATH env var override for custom binary path
• Wire the new backend into the daemon's createBackend() switch in server.go

The implementation follows the same pattern as the existing Claude CLI backend (cli.go), reusing the scanChunks splitter for streaming stdout and handling conversation history by prepending it to the prompt.

Test plan

• Set backend = "codex" in ~/.inline-cli/config.toml and verify the daemon starts with the codex backend
• With codex installed, send a query and verify streaming response works
• Verify INLINE_CLI_CODEX_PATH env var overrides the binary path
• Verify that setting an invalid backend name now lists "codex" in the supported backends error message
• Verify go build ./... passes cleanly
• Verify conversation history context is passed correctly in multi-turn sessions