Extract review logic into bot_actions module

Move `exec_review`, `download_git_diff`, and review formatting
to a new `bot_actions::review` module. Update the review flow to
post inline review comments via the Gitea API and simplify the
comment markdown to a summary. Add diff formatting that
preprocesses added lines with line numbers for the LLM prompt.

src/bot_actions/mod.rs

@@ -0,0 +1 @@
+pub mod review;