docs(readme): new ≤20-line top section — one-liners, honest recall, demo #205

Part of the 2026-10-24 launch plan (step 2).

What

The README is 1,040 lines. The first 25 lines say "download a binary and chmod it" and the strongest fact in the file — the measured recall, including the 16.7% held-out number — is at line 407. Replace the top with a ≤20-line section that leads with install one-liners, the honest numbers and a demo, and leave the rest of the file as the reference it already is.

Proposed top section (verbatim draft, adjust once crates.io/brew exist)

# injection-scanner

[CI] [CodeQL] [crates.io] [MIT]

Static scanner for prompt injection in the files your AI agent reads: `CLAUDE.md`, skill files,
MCP tool descriptions, RAG documents. Rust, one binary, no network, about 60 ms on a 40-file repo.

**84 patterns, 9 categories. Measured, not claimed:** 105/115 (91%) on the development corpus and
**2/12 (17%) on a sealed held-out set** for the newest category. Both numbers are pinned in CI and
reported side by side — [how they are measured](#how-much-does-it-actually-catch).

    cargo install injection-scanner        # or: brew install unityinflow/tap/injection-scanner
    injection-scanner check .              # exit 1 on findings; --format sarif for code scanning
    injection-scanner install-hook         # pre-commit hook: scans staged files, blocks HIGH+

![demo](docs/demo/demo.gif)

What it is: a pattern-and-structure linter with a maintained library, baselines and inline
suppression. What it is not: a classifier, a runtime guard or a guarantee — a determined attacker
can phrase around a regex. For runtime detection look at promptfoo, llm-guard or rebuff.

Also fix while there

• README.md §Pre-commit Hook pins rev: v0.0.3 in the pre-commit example — stale by three releases; make it v0.3.0.
• Add CI, Code scanning and crates.io badges (both workflows are green on main as of 2026-10-23).
• Move the current "Installation" block (binary download + chmod) below the one-liners as "Other ways to install", and keep the SHA256SUMS / gh attestation verify instructions there.

Done when

• First 20 lines contain: what it is, the two recall numbers, two install one-liners, three commands, the demo, the honest "what it is not".
• No line in the top section claims a number that tests/recall_test.rs does not pin.
• injection-scanner check README.md still exits 0 (the README documents attacks; the fences must survive).

Effort: ~2 h.