Add comprehensive .gitignore patterns for sensitive data and private keys

[Copilot]

Expands .gitignore from 146 to 320 lines to prevent accidental commits of sensitive data across modern development workflows.

Patterns Added

Cryptographic Material

• SSH private keys (id_rsa, id_ed25519, id_ecdsa, id_dsa), GPG keys (*.gpg, *.asc)
• SSL/TLS certificates (*.jks, *.bks, *.certSigningRequest)
• Excludes public keys (*.pub) - safe to track

Blockchain/Web3

• Mnemonic/seed phrases, wallet keys, smart contract deployment keys
• Geth data directories, wallet backups (metamask-backup*.json, coinbase-wallet-backup*.json)

Cloud & Infrastructure

• AWS/GCP/Azure credentials files (.aws/credentials, gcloud-credentials*.json)
• Terraform state (*.tfvars, terraform.tfstate)
• Kubernetes secrets (*-secret.yaml, kubeconfig*)
• Docker overrides and CI/CD local configs

Authentication & Secrets

• API keys (api-keys.json, *.api-key), service account keys
• Session tokens, JWT/cookie secrets, password files
• Firebase admin SDK keys, OAuth credentials

Mobile Development

• iOS provisioning profiles (*.mobileprovision), Android keystores
• Platform-specific key files (key.properties, GoogleService-Info.plist)

Backups & Cache

• Database dumps (*.dump.sql), specific backup archive patterns (backup-*.zip, *-backup.tar)
• Build caches (.turbo/, .vercel/, .netlify/)
• Excludes general archives - legitimate use case for vendor libraries

Design Decisions

• Public SSH keys remain trackable (deployment configs often need them)
• Archive patterns are specific (backup-*.zip) not broad (*.zip) to avoid breaking vendored assets
• Organized into logical sections with comments for maintainability

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