Add comprehensive docs, secret template, and README cleanup; un-ignore secret.example.txt

[jamesoncollins]

Motivation

• Reorganize and expand onboarding and developer documentation into a dedicated docs/ set and simplify the top-level README.md to point to those documents.
• Provide a safe, checked-in secret template so users can create secret.txt without accidentally committing credentials.
• Make the repository documentation easier to follow for operations, handler development, GPT tools, and testing.

Description

• Replace the verbose original README.md content with a concise quick-start and pointers to new docs, and add a QR-code linking example for signal-cli usage.
• Add multiple new documentation files: docs/ARCHITECTURE.md, docs/CONFIGURATION.md, docs/GPT_TOOLS.md, docs/HANDLERS.md, docs/OPERATIONS.md, docs/README_MIGRATION.md, and docs/TESTING.md.
• Add secret.example.txt as a safe template for local secrets and update .gitignore to explicitly un-ignore secret.example.txt via !secret.example.txt.
• Update README.md to include a brief quick-start, signal linking guidance, development/test commands, and references to the new docs.

Testing

• No automated tests were executed as part of this change.

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Codex Task