docs(design): add Cortex CLI Twitter/X launch stills

design/cli-twitter-promo/README.md

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+# Cortex CLI — Twitter/X launch stills
+
+Landscape cards for the launch tweet. All four files are **3200 × 1800 px**
+(1600 × 900 @2x, 16:9), PNG, under 5 MB — ready to attach to a post as-is.
+
+## Recommended post: `a-single-window.png`
+
+![A — single Terminal window mid-session](a-single-window.png)
+
+One clear hero: a single macOS Terminal window, mid-session. The user asked
+for rate limiting on `POST /v1/completions`; the agent is running the Shell
+tool (`npm test -- rateLimit`), two tests are already green, the follow-up
+composer sits between its dual hairlines with the green-on-cream caret, and the
+footer shows `Cortex Mini 1 · Agent · 85% context`. Largest type of the three,
+no copy to compete with the tweet text. Discreet `cortex` lockup bottom-right.
+
+Suggested alt text: *Cortex CLI running in a macOS Terminal window. A prompt
+asks for rate limiting on POST /v1/completions; the agent runs the test suite
+and two rate-limit tests pass while a third is still running.*
+
+## Alternates
+
+| File | Layout | When to use |
+|------|--------|-------------|
+| `b-layered-windows.png` | Splash card (unfocused) stacked behind the live "Working" session | Tells the start → agent-working story in one frame; slightly smaller type |
+| `c-caption-left.png` | Ink caption area left (lockup, *The coding agent CLI.*, *Plan, search, build anything.*), Terminal right | When the image has to carry the headline on its own (quote-tweets, link previews) |
+| `c-caption-left-blank.png` | Same as C with the caption area empty | Overlay your own copy in the cream `#F3EFE6` on the ink gradient |
+
+![B — stacked windows](b-layered-windows.png)
+
+![C — caption left](c-caption-left.png)
+
+## What is real
+
+Nothing in the terminals is drawn by hand. Every window shows a frame recorded
+from the signed lock TUI by `generate_tui_demo` — the same recorder that
+produces `docs/media/intro.gif` — rasterised with JetBrains Mono at 2×. The
+copy in C is product copy (the splash tagline and the composer placeholder).
+
+- Wallpaper: `docs/media/macos-wallpaper-green.jpg`
+- Window chrome: Terminal.app dark appearance (traffic lights, proxy icon, title `Cortex CLI — cortex — cols×rows`)
+- Lockup: lifted from `assets/banner.jpg`, recoloured cream
+- Palette: ink `#211F1C`, cream `#F3EFE6`, green `#1F4945` (caret and rule) — no other accent
+
+## Regenerate
+
+```bash
+sudo apt-get install -y libasound2-dev libssl-dev pkg-config   # once, for the cargo build
+python3 -m pip install pillow
+cargo build -p cortex-tui --bin generate_tui_demo
+python3 design/cli-twitter-promo/render.py            # all four stills
+python3 design/cli-twitter-promo/render.py --only a   # one of a, b, c, c-blank
+```
+
+Needs Google Chrome or Chromium on `PATH` and the JetBrains Mono + Inter fonts
+installed. Each scene's terminal size and cell width live at the top of the
+`scene_*` functions in `render.py`.