docs: update README tagline for OWASP 10/10 discoverability

* Initial plan

* Update repo tagline and subtitle for discoverability (P0)

Replace generic tagline 'Application-level security middleware for autonomous AI agents'
with Option B: 'Runtime governance for AI agents — the only toolkit covering all 10
OWASP Agentic risks with 6,100+ tests'

Also update subtitle to highlight all three key differentiators:
- Agent ACTIONS (not LLM outputs): 'Governs what agents DO, not just what they say'
- OWASP 10/10 coverage (unique to this toolkit)
- One pip install full stack

Resolves: Update repo tagline and positioning for discoverability

Co-authored-by: imran-siddique <45405841+imran-siddique@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: imran-siddique <45405841+imran-siddique@users.noreply.github.com>

Author: Copilot