docs: add package-level documentation

Per go-patterns/package-design.md, every package needs a doc comment.
Added to gitea, llm, and review packages.

gitea/client.go

@@ -1,3 +1,6 @@
+// Package gitea provides a client for the Gitea API.
+// It supports pull request operations, file content retrieval,
+// and review submission.
 package gitea
 
 import (

llm/client.go

@@ -1,3 +1,4 @@
+// Package llm provides a client for OpenAI-compatible chat completion APIs.
 package llm
 
 import (

review/prompt.go

@@ -1,3 +1,5 @@
+// Package review builds prompts for AI code review and parses LLM responses
+// into structured review results.
 package review
 
 import (