feat: context compaction (#3446)

## Compact feature:
1. Stops the model when the context window become too large
2. Add a user turn, asking for the model to summarize
3. Build a bridge that contains all the previous user message + the
summary. Rendered from a template
4. Start sampling again from a clean conversation with only that bridge

codex-rs/core/Cargo.toml

@@ -13,6 +13,7 @@ workspace = true
 
 [dependencies]
 anyhow = "1"
+askama = "0.12"
 async-channel = "2.3.1"
 base64 = "0.22"
 bytes = "1.10.1"