docs(readme): replace kill-switch tagline (TAG-2)

[jothimani-rajendran]

README.md line 7 tagline only (plan item TAG-2):

• Old: A kill switch for your Copilot agent's wrong moves.
• New: Blocks your Copilot agent's wrong moves before they land.

Surrounding <p align="center"><strong> markup is unchanged. Nothing else is touched; plugin.json descriptions are generated and update at release.

git grep -n -i "kill switch" README.md → 0 matches.

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[jothimani-rajendran]

Generated-only / verify is red, and the cause is not this PR's change. Regenerating from the catalog gives different copilot/* package files, for example verify-mcp-allowlist SKILL.md descriptions and guard script bodies. That means the catalog is ahead of the published tree. This PR only edits README.md line 7, which the regeneration diff does not touch. The same check is red on main (push run for e0ae0bf, the #10 merge) and on #10 itself.

No fix exists to port. Clearing it needs a fresh publish from the catalog, which is an owner-only release step. I'm re-running the job once for the record. actionlint and zizmor are green.

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