Add one-click installers (install.sh / install.ps1) with README docs

[Techdoll00]

What

Adds universal one-click installers — install.sh (macOS / Linux) and install.ps1 (Windows) — filling the gap the README calls out ("This repository doesn't provide a universal one-click installer"), plus a Quick Install section in both READMEs.

What the installer automates (mirrors AGENT_INSTALL.md)

1. Locate & back up — finds config.toml via CODEX_HOME/~/.codex/%USERPROFILE%\.codex, creates timestamped backups of config.toml and the model catalog.
2. Prepare install dir + vision env — copies codex-vision-proxy.py / vision_client.py to a stable dir, writes VISION_API_KEY / VISION_BASE_URL / VISION_MODEL / LANG to a 0600 env file (prompts interactively, or --non-interactive + env vars).
3. Discover the real upstream — reads the current provider base_url via tomllib and refuses to run if Codex already points at the proxy port.
4. Patch config — repoints only that provider's base_url to http://127.0.0.1:19100; nothing else in config.toml is touched (auth, wire_api, provider name all preserved).
5. Patch catalog — adds "image" to input_modalities only when it is exactly ["text"].
6. Background service — macOS LaunchAgent, Linux systemd user service, or Windows Startup .cmd; falls back to nohup when no init system is detected.
7. Verify — py_compile + port check; leaves clear "restart Codex and try view_image" instructions.

Preserves existing DeepSeek auth (the proxy forwards the original Authorization); never writes DEEPSEEK_API_KEY.

Tested

• install.sh run end-to-end against a sandbox Codex home on Ubuntu 22.04 (Python 3.12): backups created, base_url patched correctly, catalog modality patched, env written with 600 perms, guard against re-running on an already-patched config works.
• install.ps1 shares the same embedded Python logic for TOML/JSON editing (Windows-specific bits follow the AGENT_INSTALL.md Windows section).

Notes for maintainers

• .gitattributes forces LF for *.sh (protects the heredocs from CRLF corruption on Windows checkouts).
• The optional glance/ground tools are intentionally not installed by default (matching AGENT_INSTALL.md's guidance).