docs: add OpenAgentCore branding and README banner

[SaladDay]

Use the approved OpenAgentCore name in the README heading and introduction, and display the approved banner at the top. The PNG is stored in the repository with descriptive alt text so it renders directly on GitHub.

Validation: all local README links resolve, the PNG matches the approved artwork byte-for-byte, and git diff --check passes. make check passed its distribution checks, then stopped at check-database because this isolated documentation worktree has no dedicated PARSAR_AGENTS_API_TEST_DATABASE_URL; the full gate is not claimed as passing. This two-file documentation change was self-reviewed.

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