docs: replace SVG placeholders with real screenshots or demo GIF

[CodewithJha]

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Context

SVG placeholders already ship under docs/assets/ (hero, campaign, policy, tests, regressions, cli-run, storyboard). README still needs real visual proof of the product loop.

Goal

Add at least one real still (CLI or Hosted) or a short mutiny-demo.gif of init → run → test, and point README at it.

Why it matters

Placeholders prove layout; real captures prove the product. This is the highest-leverage non-code contribution for OSS discovery.

Acceptance criteria

• Add ≥1 real *.png / *.webp or docs/assets/mutiny-demo.gif (keep files reasonably sized, ideally < ~2 MB)
• Update README Screenshots section paths / alt text (prefer real media over SVG when present)
• Follow drop instructions in docs/assets/README.md (basename swap is fine)
• PR description links this issue

Suggested files

• docs/assets/* (new PNG/GIF; keep SVGs as fallback unless replacing)
• README.md (Screenshots & demo)
• docs/assets/README.md (instructions already exist)

Out of scope

• Redesigning Hosted UI, regenerating all SVGs, or large video files

Hints

• Easiest win: terminal capture of cd examples/openai_support_agent && uv run mutiny run --no-hosted → cli-run.png.
• Or record a short GIF of init → run → test per docs/assets/README.md.
• Scrub API keys / personal paths from captures before committing.

Difficulty

S · docs / media · no application code required (was catalog #4)

Docs

• docs/assets/README.md
• Catalog: GOOD_FIRST_ISSUES.md #4