Add GitHub auth command

[Adjanour]

Summary

Implement the Phase 4 authentication flow so users can run a dedicated sess auth login command that securely stores credentials and integrates with subsequent commands.

Motivation

• README.md Phase 4 roadmap and docs/technical-spec.md call for a first-class auth command.
• Relying solely on gh CLI limits users who expect in-tool authentication.

Scope

• Add a new sess auth login (or sess auth) command wired through the Cobra layer.
• Implement OAuth or PAT-based login with secure storage (OS keychain or encrypted config as documented).
• Detect and reuse existing gh authentication when available.
• Provide clear status and error handling aligned with docs/cli_design_guide.md.
• Update session workflows to respect the new auth path.

Out of Scope

• Non-GitHub identity providers (covered in future phases).
• UI for token revocation.

Acceptance Criteria

• Users can authenticate via the new command without leaving the CLI.
• Subsequent commands succeed without requiring manual gh auth login.
• Stored tokens meet the security guidance in docs/technical-spec.md.

References

• README.md (Phase 4 roadmap)
• docs/technical-spec.md
• docs/DEVELOPMENT.md (auth notes)