Performance: investigate memory issues

We have a lot of bug reports related to high memory usage and bad
performance. This PR is an attempt to identify how we can fix these
problems by creating a minimized reproduction of real-world usage
scenarios.

See `memory.test.ts` for the reproduction. In short, we use new
record/replay infrastructure to replay events from a recording of my own
coding session yesterday. When replaying these events, it's clear that
the "external" memory usage of the agent process quickly grows up to
2gb due to tree-sitter parsing that we run on every document content
change. When we disable tree-sitter parsing, the "external" memory usage
stays stable below <1mb.