feat: add shared diff-source helper (collectDiff)

Unify duplicated diff-collection logic from explain.ts, review.ts,
suggest.ts into a single collectDiff() that handles staged, unstaged,
and piped stdin sources, interactive file selection, diff truncation,
and project context collection.

Co-Authored-By: Claude <noreply@anthropic.com>

src/diff-source.ts

@@ -0,0 +1,98 @@
+// Shared diff-source helper used by explain, review, suggest, and commit commands.
+// Handles the common pattern of: detecting where to get a diff from
+// (staged, unstaged, or piped stdin), optionally presenting an interactive
+// file selector, applying truncation, and collecting project context.
+
+import { isGitRepo, getStagedFiles, getStagedDiff, getUnstagedFiles, getRepoRoot, applyFileSelection } from "./git";
+import { selectFiles } from "./selector";
+import { BACK } from "./menu";
+import { collectProjectContext } from "./context";
+import { isStdinTTY } from "./tty";
+
+export interface DiffSourceResult {
+	diff: string;
+	sourceLabel: string;
+	contextPrefix: string;
+	back: boolean;
+}
+
+const MAX_DIFF_SIZE = 15000;
+
+/**
+ * Collect a diff from the appropriate source based on flags and TTY state.
+ *
+ * - If `unstaged` is true: uses `git diff` (unstaged changes)
+ * - If TTY (interactive): shows file selector for staged/unstaged, then uses staged diff
+ * - If piped (non-TTY): reads diff from stdin
+ *
+ * Returns the diff, a human-readable source label, project context prefix,
+ * and whether the user pressed back.
+ */
+export async function collectDiff(opts: {
+	unstaged?: boolean;
+	includeProjectContext?: boolean;
+} = {}): Promise<DiffSourceResult> {
+	const { unstaged = false, includeProjectContext = true } = opts;
+
+	let diff: string;
+	let sourceLabel: string;
+	let contextPrefix = "";
+
+	if (unstaged) {
+		if (!(await isGitRepo())) {
+			throw new Error("Not a git repository.");
+		}
+		try {
+			diff = (await Bun.$`git diff`.quiet().text()).trim();
+		} catch {
+			diff = "";
+		}
+		sourceLabel = "unstaged changes";
+	} else if (isStdinTTY()) {
+		if (!(await isGitRepo())) {
+			throw new Error("Not a git repository.");
+		}
+		const stagedFiles = await getStagedFiles();
+		const unstagedFiles = await getUnstagedFiles();
+		sourceLabel = "selected changes";
+
+		if (stagedFiles.length > 0 || unstagedFiles.length > 0) {
+			const selected = await selectFiles(stagedFiles, unstagedFiles);
+			if (selected === BACK) {
+				return { diff: "", sourceLabel, contextPrefix, back: true };
+			}
+			await applyFileSelection(stagedFiles, unstagedFiles, selected);
+		}
+		diff = await getStagedDiff();
+	} else {
+		// Piped input (non-TTY)
+		const chunks: Buffer[] = [];
+		for await (const chunk of process.stdin) {
+			chunks.push(typeof chunk === "string" ? Buffer.from(chunk) : chunk);
+		}
+		diff = Buffer.concat(chunks).toString("utf-8").trim();
+		sourceLabel = "piped input";
+	}
+
+	// Truncate large diffs
+	if (diff.length > MAX_DIFF_SIZE) {
+		diff = diff.substring(0, MAX_DIFF_SIZE) + "\n... (truncated)";
+	}
+
+	// Collect project context for better AI responses
+	if (includeProjectContext && diff) {
+		try {
+			if (await isGitRepo()) {
+				const repoRoot = await getRepoRoot();
+				const ctx = await collectProjectContext(repoRoot);
+				if (ctx.packageDescription) {
+					contextPrefix = `Project: ${ctx.packageDescription}\n\n`;
+				}
+			}
+		} catch {
+			// Context collection is best-effort
+		}
+	}
+
+	return { diff, sourceLabel, contextPrefix, back: false };
+}