fix(file-reference-resolver): expand env vars in path references

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/shared/file-reference-resolver.test.ts

@@ -1,8 +1,62 @@
 import { afterAll, beforeAll, describe, expect, test } from "bun:test"
 import { mkdirSync, rmSync, symlinkSync, writeFileSync } from "node:fs"
 import { tmpdir } from "node:os"
-import { join } from "node:path"
-import { resolveFileReferencesInText } from "./file-reference-resolver"
+import { join, resolve } from "node:path"
+import { resolveFilePath, resolveFileReferencesInText } from "./file-reference-resolver"
+
+describe("resolveFilePath", () => {
+  const cwd = "/skills/gsd"
+
+  test("expands bare environment variables before resolving absolute paths", () => {
+    //#given
+    const homeDir = process.env.HOME
+    if (!homeDir) {
+      throw new Error("HOME must be set for file reference resolver tests")
+    }
+
+    //#when
+    const resolved = resolveFilePath("$HOME/foo.md", cwd)
+
+    //#then
+    expect(resolved).toBe(resolve(homeDir, "foo.md"))
+  })
+
+  test("expands braced environment variables before resolving absolute paths", () => {
+    //#given
+    const homeDir = process.env.HOME
+    if (!homeDir) {
+      throw new Error("HOME must be set for file reference resolver tests")
+    }
+
+    //#when
+    const resolved = resolveFilePath("${HOME}/foo.md", cwd)
+
+    //#then
+    expect(resolved).toBe(resolve(homeDir, "foo.md"))
+  })
+
+  test("keeps absolute paths absolute", () => {
+    //#given
+    const absolutePath = "/abs/path.md"
+
+    //#when
+    const resolved = resolveFilePath(absolutePath, cwd)
+
+    //#then
+    expect(resolved).toBe(resolve(absolutePath))
+  })
+
+  test("resolves relative paths from cwd", () => {
+    //#given
+    const relativePath = "relative/path.md"
+
+    //#when
+    const resolved = resolveFilePath(relativePath, cwd)
+
+    //#then
+    expect(resolved).toBe(resolve(cwd, relativePath))
+  })
+})
 
 describe("resolveFileReferencesInText", () => {
   const fixtureRoot = join(tmpdir(), `file-reference-resolver-${Date.now()}`)