Replace getAgent() with reading agent

[gaearon]

After the session refactor, we have a guarantee that the agent within the tree is always the same instance. Let's take advantage of that guarantee and make it clearer in the code by removing the getAgent indirection.

Before:

  const {getAgent} = useAgent()
  // usage
  getAgent().foo.bar()

After:

  const {agent} = useAgent()
  // usage
  agent.foo.bar()

## Test Plan

See session/index.tsx, notice that there's no actual change in semantics. The value was already being captured in a closure.

Then read through the code. Most of this is simple find-and-replace, with some manual edits to fix types. I relied on types to fix the rest.

Do some basic things in the app, switch accounts, verify that still works.

[render]

Your Render PR Server URL is https://social-app-pr-4243.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-cpav3oq1hbls73dim9t0.

[github-actions]

Old size	New size	Diff
7.3 MB	7.3 MB	-420 B (-0.01%)

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[haileyok]

cmd+f'd the codebase, don't see any other getAgent calls. Also clicked around a bit and didn't have any issues, seems good to me.