fix: resolve symlinks in Filesystem.resolve() to prevent duplicate Instance contexts

Filesystem.resolve() used path.resolve() which normalizes path segments
but does not resolve symlinks. When opencode runs from a symlinked
directory, Instance.provide() could create duplicate contexts for the
same physical directory, causing Bus event isolation and a blank TUI.

Move symlink resolution (realpathSync) into Filesystem.resolve() so all
callers benefit. Falls back to the unresolved path if the target does
not exist, matching the guard pattern in normalizePath().

Fixes #16647
Fixes #15482

Author: jmylchreest

packages/opencode/src/util/filesystem.ts

@@ -114,8 +114,15 @@ export namespace Filesystem {
   }
 
   // We cannot rely on path.resolve() here because git.exe may come from Git Bash, Cygwin, or MSYS2, so we need to translate these paths at the boundary.
+  // Also resolves symlinks so that callers using the result as a cache key
+  // always get the same canonical path for a given physical directory.
   export function resolve(p: string): string {
-    return normalizePath(pathResolve(windowsPath(p)))
+    const resolved = pathResolve(windowsPath(p))
+    try {
+      return normalizePath(realpathSync(resolved))
+    } catch {
+      return normalizePath(resolved)
+    }
   }
 
   export function windowsPath(p: string): string {

packages/opencode/test/util/filesystem.test.ts

@@ -502,5 +502,23 @@ describe("filesystem", () => {
       const drive = tmp.path[0].toLowerCase()
       expect(Filesystem.resolve(`/mnt/${drive}`)).toBe(Filesystem.resolve(`${drive.toUpperCase()}:/`))
     })
+
+    test("resolves symlinked directory to canonical path", async () => {
+      if (process.platform === "win32") return
+      await using tmp = await tmpdir()
+      const link = tmp.path + "-symlink"
+      await fs.symlink(tmp.path, link)
+      try {
+        expect(Filesystem.resolve(link)).toBe(Filesystem.resolve(tmp.path))
+      } finally {
+        await fs.unlink(link).catch(() => {})
+      }
+    })
+
+    test("returns unresolved path when target does not exist", () => {
+      const nonExistent = "/tmp/opencode-test-does-not-exist-" + Date.now()
+      const result = Filesystem.resolve(nonExistent)
+      expect(result).toBe(nonExistent)
+    })
   })
 })