Feat: leverage secrets store agent for session data

[major0]

Currently we are leveraging a simple file store for saving session data, which works in a pinch, but it is widely inappropriate from a security perspective; we should be leveraging a secrets store.

The secrets store should:

• Be configurable: allow users to specify a custom store, such as a PGP/GPG Agent
• Default to leveraging the local OS's default store, if available.

Refs:

• https://github.com/keybase/go-keychain
• https://github.com/prep/gpg