support install-wide defaults for provider config

[sentry-junior]

add install-wide defaults for conversation/provider config, e.g. defaulting sentry.org to sentry across a workspace/install instead of only per-channel defaults.

context:

• raised from a slack thread after junior over-indexed on its own project/org context for a sentry lookup
• current workaround appears to be per-channel defaults
• desired behavior is a higher-level default for an install/workspace/org so common internal contexts do not need to be reconfigured channel by channel

examples:

• default sentry.org to sentry for the Sentry install
• potentially apply the same pattern to other provider config keys where per-install defaults make sense

notes:

• this should reduce wrong-target lookups and improve first-try routing for internal usage
• user framing in thread: "install-wide defaults (e.g. we wanna default sentry.org to sentry for us)"