fastapi-explain

## Target `$ARGUMENTS` If `$ARGUMENTS` is empty, explain the current branch diff using the dump below. Otherwise, treat `$ARGUMENTS` as the target — a file path, function name, or concept — and explain that. ## Current branch diff (used when target is empty) Run `git diff master...HEAD` and use its output. ## Instructions **If the target is empty (no arguments):** 1. The diff above shows everything changed on this branch. If it's empty, fall back to `git diff` (unstaged) and `git diff --cached` (staged). 2. Read the full files involved to understand the surrounding context — do not paraphrase from the diff alone. 3. Explain what changed and why, covering: - The purpose of the changes as a whole - How the modified components interact - Any non-obvious behavior or edge cases introduced **If the target is provided:** 1. Read CLAUDE.md and any matching `.claude/rules/*.md` for the area the target lives in. 2. Find the relevant code using Grep and Glob. 3. Read the full files involved to understand context. 4. Trace the call chain and data flow end-to-end. 5. Explain how it works in plain language, covering: - What it does and why it exists - Key components and how they interact - Important design decisions or trade-offs - Any non-obvious behavior or edge cases ## Rules - Tailor the depth to the question — "what does this function do" needs less than "how does auth work". - Use concrete examples from the code, not abstract descriptions. - Cite file:li...