Enhance .gitignore to protect sensitive data and private keys

[Copilot]

Blockchain repositories require comprehensive protection against accidental commits of credentials, private keys, and sensitive configuration. The existing .gitignore had basic coverage but lacked patterns for common sensitive file types.

Changes

Added 83+ patterns targeting:

• Private Keys: SSH keys (RSA, DSA, ECDSA, ED25519), SSL keystores (*.p12, *.pfx, *.jks), PEM files
• Cryptocurrency: Wallet files, keystore/wallet directories at any level, mnemonic/seed phrases
• API Credentials: Pattern pairs for both prefix (apikey*.json) and contains (*apikey*.json) matching with specific extensions (json, txt, yml, yaml, env)
• Cloud Providers: AWS credentials, GCP service accounts (*service-account-*.json), Azure configs
• Database: Flexible patterns (database*.yml, *connection-string*.{json,txt,yml,yaml,env})
• Environment: .env.* files and secrets patterns

Design Decisions

Patterns use specific file extensions rather than wildcards alone (e.g., *credentials*.json vs *credentials*) to avoid false positives on source files like credentials_validator.py. Directory patterns use **/keystore/ to match at any repository level with override guidance in comments.

Both prefix and contains patterns for API keys are kept despite technical redundancy for explicitness around common naming conventions.

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