docs: refresh README for platform and contributor clarity

[diese-tech]

Summary

Refreshes the root README to make SALBot's role in the SAL platform clearer and easier to scan.

• leads with current product identity and status;
• clarifies sal-site / sal-database ownership boundaries;
• summarizes shipped workflows without duplicating command docs;
• keeps architecture and validation principles visible;
• routes deeper setup and operational material into existing docs;
• preserves the repository's source-available/non-OSS license posture.

No runtime behavior changes.

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