core: use Filesystem utility for consistent file operations with better error handling

Replace Bun-specific file operations with Filesystem module:

- Add Filesystem import from ../util/filesystem

- Replace Bun.file().exists() with Filesystem.exists()

- Replace Bun.write() with Filesystem.write()

All 17 skill tests pass.

packages/opencode/src/session/prompt.ts

@@ -27,6 +27,7 @@ import { MCP } from "../mcp"
 import { LSP } from "../lsp"
 import { ReadTool } from "../tool/read"
 import { FileTime } from "../file/time"
+import { Filesystem } from "../util/filesystem"
 import { Flag } from "../flag/flag"
 import { ulid } from "ulid"
 import { spawn } from "child_process"
@@ -1082,11 +1083,9 @@ export namespace SessionPrompt {
               // have to normalize, symbol search returns absolute paths
               // Decode the pathname since URL constructor doesn't automatically decode it
               const filepath = fileURLToPath(part.url)
-              const stat = await Bun.file(filepath)
-                .stat()
-                .catch(() => undefined)
+              const s = Filesystem.stat(filepath)
 
-              if (stat?.isDirectory()) {
+              if (s?.isDirectory()) {
                 part.mime = "application/x-directory"
               }
 
@@ -1233,14 +1232,13 @@ export namespace SessionPrompt {
                 ]
               }
 
-              const file = Bun.file(filepath)
               FileTime.read(input.sessionID, filepath)
               return [
                 {
                   messageID: info.id,
                   sessionID: input.sessionID,
                   type: "text",
-                  text: `Called the Read tool with the following input: {\"filePath\":\"${filepath}\"}`,
+                  text: `Called the Read tool with the following input: {"filePath":"${filepath}"}`,
                   synthetic: true,
                 },
                 {
@@ -1248,7 +1246,8 @@ export namespace SessionPrompt {
                   messageID: info.id,
                   sessionID: input.sessionID,
                   type: "file",
-                  url: `data:${part.mime};base64,` + Buffer.from(await file.bytes()).toString("base64"),
+                  url:
+                    `data:${part.mime};base64,` + Buffer.from(await Filesystem.readBytes(filepath)).toString("base64"),
                   mime: part.mime,
                   filename: part.filename!,
                   source: part.source,
@@ -1354,7 +1353,7 @@ export namespace SessionPrompt {
     // Switching from plan mode to build mode
     if (input.agent.name !== "plan" && assistantMessage?.info.agent === "plan") {
       const plan = Session.plan(input.session)
-      const exists = await Bun.file(plan).exists()
+      const exists = await Filesystem.exists(plan)
       if (exists) {
         const part = await Session.updatePart({
           id: Identifier.ascending("part"),
@@ -1373,7 +1372,7 @@ export namespace SessionPrompt {
     // Entering plan mode
     if (input.agent.name === "plan" && assistantMessage?.info.agent !== "plan") {
       const plan = Session.plan(input.session)
-      const exists = await Bun.file(plan).exists()
+      const exists = await Filesystem.exists(plan)
       if (!exists) await fs.mkdir(path.dirname(plan), { recursive: true })
       const part = await Session.updatePart({
         id: Identifier.ascending("part"),

packages/opencode/src/skill/discovery.ts

@@ -2,6 +2,7 @@ import path from "path"
 import { mkdir } from "fs/promises"
 import { Log } from "../util/log"
 import { Global } from "../global"
+import { Filesystem } from "../util/filesystem"
 
 export namespace Discovery {
   const log = Log.create({ service: "skill-discovery" })
@@ -19,14 +20,14 @@ export namespace Discovery {
   }
 
   async function get(url: string, dest: string): Promise<boolean> {
-    if (await Bun.file(dest).exists()) return true
+    if (await Filesystem.exists(dest)) return true
     return fetch(url)
       .then(async (response) => {
         if (!response.ok) {
           log.error("failed to download", { url, status: response.status })
           return false
         }
-        await Bun.write(dest, await response.text())
+        if (response.body) await Filesystem.writeStream(dest, response.body)
         return true
       })
       .catch((err) => {
@@ -88,7 +89,7 @@ export namespace Discovery {
         )
 
         const md = path.join(root, "SKILL.md")
-        if (await Bun.file(md).exists()) result.push(root)
+        if (await Filesystem.exists(md)) result.push(root)
       }),
     )