feat(hooks): add hashline-edit-diff-enhancer for TUI inline diff display

Capture file content before hashline edit execution and compute filediff
metadata after, enabling opencode TUI to render inline diffs for the
plugin's edit tool (which replaces the built-in EditTool).

src/hooks/hashline-edit-diff-enhancer/hook.ts

@@ -0,0 +1,129 @@
+import { log } from "../../shared"
+
+interface HashlineEditDiffEnhancerConfig {
+	hashline_edit?: { enabled: boolean }
+}
+
+type BeforeInput = { tool: string; sessionID: string; callID: string }
+type BeforeOutput = { args: Record<string, unknown> }
+type AfterInput = { tool: string; sessionID: string; callID: string }
+type AfterOutput = { title: string; output: string; metadata: Record<string, unknown> }
+
+const STALE_TIMEOUT_MS = 5 * 60 * 1000
+
+const pendingCaptures = new Map<string, { content: string; filePath: string; storedAt: number }>()
+
+function makeKey(sessionID: string, callID: string): string {
+	return `${sessionID}:${callID}`
+}
+
+function cleanupStaleEntries(): void {
+	const now = Date.now()
+	for (const [key, entry] of pendingCaptures) {
+		if (now - entry.storedAt > STALE_TIMEOUT_MS) {
+			pendingCaptures.delete(key)
+		}
+	}
+}
+
+function isEditTool(toolName: string): boolean {
+	return toolName === "edit"
+}
+
+function countLineDiffs(oldContent: string, newContent: string): { additions: number; deletions: number } {
+	const oldLines = oldContent.split("\n")
+	const newLines = newContent.split("\n")
+
+	const oldSet = new Map<string, number>()
+	for (const line of oldLines) {
+		oldSet.set(line, (oldSet.get(line) ?? 0) + 1)
+	}
+
+	const newSet = new Map<string, number>()
+	for (const line of newLines) {
+		newSet.set(line, (newSet.get(line) ?? 0) + 1)
+	}
+
+	let deletions = 0
+	for (const [line, count] of oldSet) {
+		const newCount = newSet.get(line) ?? 0
+		if (count > newCount) {
+			deletions += count - newCount
+		}
+	}
+
+	let additions = 0
+	for (const [line, count] of newSet) {
+		const oldCount = oldSet.get(line) ?? 0
+		if (count > oldCount) {
+			additions += count - oldCount
+		}
+	}
+
+	return { additions, deletions }
+}
+
+async function captureOldContent(filePath: string): Promise<string> {
+	try {
+		const file = Bun.file(filePath)
+		if (await file.exists()) {
+			return await file.text()
+		}
+	} catch {
+		log("[hashline-edit-diff-enhancer] failed to read old content", { filePath })
+	}
+	return ""
+}
+
+export function createHashlineEditDiffEnhancerHook(config: HashlineEditDiffEnhancerConfig) {
+	const enabled = config.hashline_edit?.enabled ?? false
+
+	return {
+		"tool.execute.before": async (input: BeforeInput, output: BeforeOutput) => {
+			if (!enabled || !isEditTool(input.tool)) return
+
+			const filePath = typeof output.args.path === "string" ? output.args.path : undefined
+			if (!filePath) return
+
+			cleanupStaleEntries()
+			const oldContent = await captureOldContent(filePath)
+			pendingCaptures.set(makeKey(input.sessionID, input.callID), {
+				content: oldContent,
+				filePath,
+				storedAt: Date.now(),
+			})
+		},
+
+		"tool.execute.after": async (input: AfterInput, output: AfterOutput) => {
+			if (!enabled || !isEditTool(input.tool)) return
+
+			const key = makeKey(input.sessionID, input.callID)
+			const captured = pendingCaptures.get(key)
+			if (!captured) return
+			pendingCaptures.delete(key)
+
+			const { content: oldContent, filePath } = captured
+
+			let newContent: string
+			try {
+				newContent = await Bun.file(filePath).text()
+			} catch {
+				log("[hashline-edit-diff-enhancer] failed to read new content", { filePath })
+				return
+			}
+
+			const { additions, deletions } = countLineDiffs(oldContent, newContent)
+
+			output.metadata.filediff = {
+				file: filePath,
+				path: filePath,
+				before: oldContent,
+				after: newContent,
+				additions,
+				deletions,
+			}
+
+			output.title = filePath
+		},
+	}
+}