fix(skill-loader): discover skills from .agents/skills/ directory

Add discoverProjectAgentsSkills() for project-level .agents/skills/ and
discoverGlobalAgentsSkills() for ~/.agents/skills/ — matching OpenCode's
native skill discovery paths (https://opencode.ai/docs/skills/).

Updated discoverAllSkills(), discoverSkills(), and createSkillContext()
to include these new sources with correct priority ordering.

Co-authored-by: dtateks <dtateks@users.noreply.github.com>
Closes #1818

src/features/opencode-skill-loader/agents-skills-global.test.ts

@@ -0,0 +1,48 @@
+import { describe, it, expect, beforeEach, afterEach, mock } from "bun:test"
+import { mkdirSync, writeFileSync, rmSync } from "fs"
+import { join } from "path"
+import { tmpdir } from "os"
+
+const TEST_DIR = join(tmpdir(), "agents-global-skills-test-" + Date.now())
+const TEMP_HOME = join(TEST_DIR, "home")
+
+describe("discoverGlobalAgentsSkills", () => {
+  beforeEach(() => {
+    mkdirSync(TEST_DIR, { recursive: true })
+    mkdirSync(TEMP_HOME, { recursive: true })
+  })
+
+  afterEach(() => {
+    mock.restore()
+    rmSync(TEST_DIR, { recursive: true, force: true })
+  })
+
+  it("#given a skill in ~/.agents/skills/ #when discoverGlobalAgentsSkills is called #then it discovers the skill", async () => {
+    //#given
+    const skillContent = `---
+name: agent-global-skill
+description: A skill from global .agents/skills directory
+---
+Skill body.
+`
+    const agentsGlobalSkillsDir = join(TEMP_HOME, ".agents", "skills")
+    const skillDir = join(agentsGlobalSkillsDir, "agent-global-skill")
+    mkdirSync(skillDir, { recursive: true })
+    writeFileSync(join(skillDir, "SKILL.md"), skillContent)
+
+    mock.module("os", () => ({
+      homedir: () => TEMP_HOME,
+      tmpdir,
+    }))
+
+    //#when
+    const { discoverGlobalAgentsSkills } = await import("./loader")
+    const skills = await discoverGlobalAgentsSkills()
+    const skill = skills.find(s => s.name === "agent-global-skill")
+
+    //#then
+    expect(skill).toBeDefined()
+    expect(skill?.scope).toBe("user")
+    expect(skill?.definition.description).toContain("A skill from global .agents/skills directory")
+  })
+})