feat(write-existing-file-guard): add hook to prevent write tool from overwriting existing files

Adds a PreToolUse hook that intercepts write operations and throws an error
if the target file already exists, guiding users to use the edit tool instead.

- Throws error: 'File already exists. Use edit tool instead.'
- Hook is enabled by default, can be disabled via disabled_hooks
- Includes comprehensive test suite with BDD-style comments

src/hooks/write-existing-file-guard/index.ts

@@ -0,0 +1,33 @@
+import type { Hooks, PluginInput } from "@opencode-ai/plugin"
+import { existsSync } from "fs"
+import { resolve, isAbsolute } from "path"
+import { log } from "../../shared"
+
+export function createWriteExistingFileGuardHook(ctx: PluginInput): Hooks {
+  return {
+    "tool.execute.before": async (input, output) => {
+      const toolName = input.tool?.toLowerCase()
+      if (toolName !== "write") {
+        return
+      }
+
+      const args = output.args as { filePath?: string; path?: string; file_path?: string } | undefined
+      const filePath = args?.filePath ?? args?.path ?? args?.file_path
+      if (!filePath) {
+        return
+      }
+
+      const resolvedPath = isAbsolute(filePath) ? filePath : resolve(ctx.directory, filePath)
+
+      if (existsSync(resolvedPath)) {
+        log("[write-existing-file-guard] Blocking write to existing file", {
+          sessionID: input.sessionID,
+          filePath,
+          resolvedPath,
+        })
+
+        throw new Error("File already exists. Use edit tool instead.")
+      }
+    },
+  }
+}