fix: Pick a session by endpoint in auth switch

[scotwells]

Summary

When one email is signed in to production and staging, switching accounts always opened a picker, so a script, CI job or agent had no way to choose between them.

You can now name the environment with --endpoint, using the value datumctl auth list prints, or pass the full session name:

$ datumctl auth list
User              Endpoint                   Status
swells@datum.net  api.datum.net              Active
swells@datum.net  api.staging.env.datum.net

$ datumctl auth switch swells@datum.net --endpoint api.staging.env.datum.net
✓ Switched to Scot Wells (swells@datum.net)
  Context:  datum
  Endpoint: api.staging.env.datum.net

Without a terminal, an ambiguous switch tells you exactly what to run instead of failing with no way forward:

$ datumctl auth switch swells@datum.net
error: swells@datum.net is signed in on more than one endpoint. Choosing one interactively requires a terminal.
Run one of:
  datumctl auth switch swells@datum.net --endpoint api.datum.net
  datumctl auth switch swells@datum.net --endpoint api.staging.env.datum.net

$ datumctl auth switch swells@datum.net --endpoint api.nope.example
error: No session for swells@datum.net on endpoint api.nope.example.
Signed-in endpoints: api.datum.net, api.staging.env.datum.net

In a terminal the picker works as before, and when datumctl ctx use points you at a context in your other environment, its suggested switch command now includes the endpoint.

Test plan

• An email on two endpoints switches to each by endpoint and back, and the active context follows every time
• An unknown endpoint fails and lists the endpoints you are signed in on
• With no terminal, an ambiguous switch lists one runnable command per session
• Switching leaves your other sessions signed in

Fixes #277

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