fix: use fs.realpath instead of manual path.resolve for symlink resolution

resolveSymlink and resolveSymlinkAsync incorrectly resolved relative
symlinks by using path.resolve(filePath, '..', linkTarget). This fails
when symlinks use multi-level relative paths (e.g. ../../skills/...) or
when symlinks are chained (symlink pointing to a directory containing
more symlinks).

Replace with fs.realpathSync/fs.realpath which delegates to the OS for
correct resolution of all symlink types: relative, absolute, chained,
and nested.

Fixes #1738

AI-assisted-by: claude-opus-4.6 via opencode
AI-contribution: partial
AI-session: 20260212-120629-4gTXvDGV

src/shared/file-utils.test.ts

@@ -0,0 +1,103 @@
+import { describe, it, expect, beforeAll, afterAll } from "bun:test"
+import { mkdirSync, writeFileSync, symlinkSync, rmSync } from "fs"
+import { join } from "path"
+import { tmpdir } from "os"
+import { resolveSymlink, resolveSymlinkAsync, isSymbolicLink } from "./file-utils"
+
+const testDir = join(tmpdir(), "file-utils-test-" + Date.now())
+
+// Create a directory structure that mimics the real-world scenario:
+//
+//   testDir/
+//   ├── repo/
+//   │   ├── skills/
+//   │   │   └── category/
+//   │   │       └── my-skill/
+//   │   │           └── SKILL.md
+//   │   └── .opencode/
+//   │       └── skills/
+//   │           └── my-skill -> ../../skills/category/my-skill  (relative symlink)
+//   └── config/
+//       └── skills -> ../repo/.opencode/skills                  (absolute symlink)
+
+const realSkillDir = join(testDir, "repo", "skills", "category", "my-skill")
+const repoOpencodeSkills = join(testDir, "repo", ".opencode", "skills")
+const configSkills = join(testDir, "config", "skills")
+
+beforeAll(() => {
+	// Create real skill directory with a file
+	mkdirSync(realSkillDir, { recursive: true })
+	writeFileSync(join(realSkillDir, "SKILL.md"), "# My Skill")
+
+	// Create .opencode/skills/ with a relative symlink to the real skill
+	mkdirSync(repoOpencodeSkills, { recursive: true })
+	symlinkSync("../../skills/category/my-skill", join(repoOpencodeSkills, "my-skill"))
+
+	// Create config/skills as an absolute symlink to .opencode/skills
+	mkdirSync(join(testDir, "config"), { recursive: true })
+	symlinkSync(repoOpencodeSkills, configSkills)
+})
+
+afterAll(() => {
+	rmSync(testDir, { recursive: true, force: true })
+})
+
+describe("resolveSymlink", () => {
+	it("resolves a regular file path to itself", () => {
+		const filePath = join(realSkillDir, "SKILL.md")
+		expect(resolveSymlink(filePath)).toBe(filePath)
+	})
+
+	it("resolves a relative symlink to its real path", () => {
+		const symlinkPath = join(repoOpencodeSkills, "my-skill")
+		expect(resolveSymlink(symlinkPath)).toBe(realSkillDir)
+	})
+
+	it("resolves a chained symlink (symlink-to-dir-containing-symlinks) to the real path", () => {
+		// This is the real-world scenario:
+		// config/skills/my-skill -> (follows config/skills) -> repo/.opencode/skills/my-skill -> repo/skills/category/my-skill
+		const chainedPath = join(configSkills, "my-skill")
+		expect(resolveSymlink(chainedPath)).toBe(realSkillDir)
+	})
+
+	it("returns the original path for non-existent paths", () => {
+		const fakePath = join(testDir, "does-not-exist")
+		expect(resolveSymlink(fakePath)).toBe(fakePath)
+	})
+})
+
+describe("resolveSymlinkAsync", () => {
+	it("resolves a regular file path to itself", async () => {
+		const filePath = join(realSkillDir, "SKILL.md")
+		expect(await resolveSymlinkAsync(filePath)).toBe(filePath)
+	})
+
+	it("resolves a relative symlink to its real path", async () => {
+		const symlinkPath = join(repoOpencodeSkills, "my-skill")
+		expect(await resolveSymlinkAsync(symlinkPath)).toBe(realSkillDir)
+	})
+
+	it("resolves a chained symlink (symlink-to-dir-containing-symlinks) to the real path", async () => {
+		const chainedPath = join(configSkills, "my-skill")
+		expect(await resolveSymlinkAsync(chainedPath)).toBe(realSkillDir)
+	})
+
+	it("returns the original path for non-existent paths", async () => {
+		const fakePath = join(testDir, "does-not-exist")
+		expect(await resolveSymlinkAsync(fakePath)).toBe(fakePath)
+	})
+})
+
+describe("isSymbolicLink", () => {
+	it("returns true for a symlink", () => {
+		expect(isSymbolicLink(join(repoOpencodeSkills, "my-skill"))).toBe(true)
+	})
+
+	it("returns false for a regular directory", () => {
+		expect(isSymbolicLink(realSkillDir)).toBe(false)
+	})
+
+	it("returns false for a non-existent path", () => {
+		expect(isSymbolicLink(join(testDir, "does-not-exist"))).toBe(false)
+	})
+})