feat(infra): Multi-account support with context switching #113

Summary

Support multiple Google accounts with context switching.

Priority: P2 | Complexity: Complex

Commands

gws context add work --client-id=xxx
gws context add personal --client-id=yyy
gws context use work
# or: GWS_CONTEXT=personal gws gmail list

Motivation

Similar to jk context use, bkt context create, and gogcli's email alias system. Users with multiple Google accounts (work + personal) need easy switching.

Implementation

• Store multiple token sets, keyed by context name
• gws context add/use/list/remove subcommands
• GWS_CONTEXT env var for per-command override
• Config stored in ~/.config/gws/contexts/
• Default context persisted in config