[codex] Remove legacy device authentication

[gemcoder21]

Summary

• remove support for legacy x-device-* and x-wallet-id request authentication headers
• require wallet-scoped device auth to use the Authorization: Gem ... payload wallet id
• simplify Gem auth parsing and update device/wallet authentication docs

Validation

• just format
• cargo clippy -p gem_auth -- -D warnings
• cargo clippy -p storage -- -D warnings
• cargo clippy -p api -- -D warnings
• just test storage
• just test gem_auth
• just test api

[gemini-code-assist]

Code Review

This pull request removes legacy device authentication mechanisms, including individual x-device-* and x-wallet-id headers, in favor of a single unified Authorization: Gem ... header. It cleans up the authentication logic across the API, simplifies signature verification, removes the legacy fallback code, and updates the corresponding documentation.