feat: add opt-in session list caching with stale-while-revalidate

[joshmedeski]

Note

this is an experimental opt-in caching feature that will speed up load times for sesh

Summary

• Add file-based cache layer (cache/ package) using gob encoding at $XDG_CACHE_HOME/sesh/sessions.gob
• Add CachingLister decorator with stale-while-revalidate strategy (5s soft TTL) that wraps the existing lister
• Wire caching into deps.go behind a cache = true config flag, with automatic refresh after sesh connect

Why

Repeated sesh list / sesh pick calls can be slow when fetching from tmux, zoxide, and config sources. Caching returns instant results for subsequent calls while keeping data fresh via background revalidation.

Notes

• Caching is opt-in and disabled by default -- no behavior change without cache = true in sesh.toml
• Cache stores the full unfiltered session list; view-level filters like HideAttached are applied after cache read
• Atomic writes via temp file + rename to prevent corruption
• Comprehensive tests for both cache/ and lister/ packages

cache = true