fix(webapp): keep the dashboard agent's tool calls on the user's instance (#4740)

## Summary

Follow-up to #4738. Splits the dashboard agent's base URL into two: the
instance that hosts the agent project (used for sessions), and the
instance the agent acts against as the user (used by its read-tools).
#4738 only needed the first, but moved the second along with it, which
breaks the tools when the agent runs on a different instance than the
webapp.

## Root cause

The agent's read-tools call the API as the logged-in user via a
delegated user-actor token. The webapp signs that token with its own
`SESSION_SECRET`, scoped to its own `userId` and `environmentId`, so it
can only be verified by, and only resolves the user's data on, that same
instance. #4738 routed the injected `apiOrigin` those tools use to the
agent's host instance, so the token no longer verifies and the data
isn't there.

## Fix

`dashboardAgentApiOrigin()` stays the agent's host instance (sessions,
task triggers, realtime, the `in` forward). A new
`dashboardAgentUserApiOrigin()` returns the webapp's own origin
(`API_ORIGIN ?? APP_ORIGIN`) and is injected into the run metadata the
tools use. Same-instance deployments resolve both to the same host, so
behavior is unchanged there.