feat(code): friendly errors, batch history, persistent command history

- Agent errors sanitized: raw hyper/h2/gRPC dumps replaced with
  human-readable messages ("sol disconnected", "connection lost", etc.)
- Batch history loading: single viewport rebuild instead of per-entry
- Persistent command history: saved to .sunbeam/history, loaded on start
- Default model: mistral-medium-latest (personality adherence)

.sunbeam/history

@@ -0,0 +1,35 @@
+hmm
+just testing the ux
+/exit
+/exit
+hmm, scrolling is very slow. needs to be async
+/exit
+/exit
+/exit
+/exit
+/exit
+/exit
+/exit
+[<35;52;20M/exit
+/exit
+/exit
+hey you
+who are you?
+hmm.
+that's not right.
+you're supposed to be `sol`
+what's on page 2 of hackernews today?
+don't you have fetch tooling?
+/exit
+hey.
+hey
+/exot
+/exit
+hey
+say hello from sunbeam code!
+can you search the web for me and tell me what's on page 2 of hackernews?
+/exit
+hey boo
+tell me about yourself
+can you please do some googling and some research to see if i can use devstral-medium as an agent?
+/exit