docs(readme): add CI, version, license, and supported-agent badges

[mattmaynes]

Adds a badge block under the tagline in the README.

What

• Row 1 — CI (GitHub-native workflow badge, main), version (shields.io/github/v/release), License: MIT.
• Row 2 — supported-agent badges: Claude Code, OpenAI Codex, Gemini CLI, Cursor (each links to Quick start).

Notes

• The version badge reads the latest GitHub Release. It renders empty until feat(release): sync plugin version via VERSION + auto tag & GitHub Release on merge #16 lands and the first release/tag exists, then tracks the bare-semver tag that PR's release job mints.
• OpenAI's logo was removed from simple-icons (slug 404s), so the Codex badge uses the OpenAI brand color without an icon; the other three carry their logos.
• All badge URLs verified to resolve (200) and logos confirmed to embed.

Verification

• ./test.sh → PASS (incl. README token-drift check — badges don't touch token figures).
• Pure-docs change, no behavior change → self-review per protocol §5.4.

🤖 Generated with Claude Code