feat(skills): compact skill paths with ~ to reduce prompt tokens

Replace absolute home directory prefix with ~ in skill <location> tags
injected into the system prompt. Models understand ~ expansion and the
read tool resolves it, so this is a safe, backward-compatible change.

Saves ~5-6 tokens per skill path. For a workspace with 90+ skills,
this reduces system prompt size by ~400-600 tokens.

Changes:
- Add compactSkillPaths() helper in workspace.ts
- Apply in buildWorkspaceSkillSnapshot and buildWorkspaceSkillsPrompt
- Add test for path compaction behavior

Before: /Users/alice/.bun/install/global/node_modules/openclaw/skills/github/SKILL.md
After:  ~/.bun/install/global/node_modules/openclaw/skills/github/SKILL.md

src/agents/skills.compact-skill-paths.test.ts

@@ -0,0 +1,80 @@
+import fs from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+import { describe, expect, it } from "vitest";
+import { buildWorkspaceSkillsPrompt } from "./skills.js";
+
+async function writeSkill(params: {
+  dir: string;
+  name: string;
+  description: string;
+  body?: string;
+}) {
+  const { dir, name, description, body } = params;
+  await fs.mkdir(dir, { recursive: true });
+  await fs.writeFile(
+    path.join(dir, "SKILL.md"),
+    `---
+name: ${name}
+description: ${description}
+---
+
+${body ?? `# ${name}\n`}
+`,
+    "utf-8",
+  );
+}
+
+describe("compactSkillPaths", () => {
+  it("replaces home directory prefix with ~ in skill locations", async () => {
+    const workspaceDir = await fs.mkdtemp(path.join(os.tmpdir(), "openclaw-compact-"));
+    const skillDir = path.join(workspaceDir, "skills", "test-skill");
+
+    await writeSkill({
+      dir: skillDir,
+      name: "test-skill",
+      description: "A test skill for path compaction",
+    });
+
+    const prompt = buildWorkspaceSkillsPrompt(workspaceDir, {
+      bundledSkillsDir: path.join(workspaceDir, ".bundled-empty"),
+      managedSkillsDir: path.join(workspaceDir, ".managed-empty"),
+    });
+
+    const home = os.homedir();
+    // The prompt should NOT contain the absolute home directory path
+    // when the skill is under the home directory (which tmpdir usually is on macOS)
+    if (workspaceDir.startsWith(home)) {
+      expect(prompt).not.toContain(home + path.sep);
+      expect(prompt).toContain("~/");
+    }
+
+    // The skill name and description should still be present
+    expect(prompt).toContain("test-skill");
+    expect(prompt).toContain("A test skill for path compaction");
+
+    await fs.rm(workspaceDir, { recursive: true, force: true });
+  });
+
+  it("preserves paths outside home directory", async () => {
+    // Skills outside ~ should keep their absolute paths
+    const workspaceDir = await fs.mkdtemp(path.join(os.tmpdir(), "openclaw-compact-"));
+    const skillDir = path.join(workspaceDir, "skills", "ext-skill");
+
+    await writeSkill({
+      dir: skillDir,
+      name: "ext-skill",
+      description: "External skill",
+    });
+
+    const prompt = buildWorkspaceSkillsPrompt(workspaceDir, {
+      bundledSkillsDir: path.join(workspaceDir, ".bundled-empty"),
+      managedSkillsDir: path.join(workspaceDir, ".managed-empty"),
+    });
+
+    // Should still contain a valid location tag
+    expect(prompt).toMatch(/<location>[^<]+SKILL\.md<\/location>/);
+
+    await fs.rm(workspaceDir, { recursive: true, force: true });
+  });
+});