reactor(cli): move memoh cli to tui

1. Split the oversized `cmd/agent` entrypoint into a multi-file package and update dev/build scripts to use the package path instead of compiling `main.go` directly.
2. Add a new `memoh` terminal UI for local bot chat, with Bubble Tea

cmd/agent/mise.toml

@@ -4,5 +4,5 @@ dir = "{{cwd}}"
 [tasks.start]
 alias = "dev"
 description = "Start server"
-run = "go run cmd/agent/main.go"
+run = "go run cmd/agent"
 depends = ["//:go-install"]