Add the ability to follow the agent as it makes edits (#29839)

Nathan here: I also tacked on a bunch of UI refinement.

Release Notes:

- Introduced the ability to follow the agent around as it reads and
edits files.

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Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/clock/src/clock.rs

@@ -10,6 +10,7 @@ use std::{
 pub use system_clock::*;
 
 pub const LOCAL_BRANCH_REPLICA_ID: u16 = u16::MAX;
+pub const AGENT_REPLICA_ID: u16 = u16::MAX - 1;
 
 /// A unique identifier for each distributed node.
 pub type ReplicaId = u16;