fix(opencode): use correct global config path ~/.config/opencode (#117)

The OpenCode installer was writing to ~/.opencode but OpenCode expects
global configuration at ~/.config/opencode per XDG Base Directory spec.

Fixes:
- src/commands/install.ts: Change default output from ~/.opencode to
  ~/.config/opencode
- src/targets/opencode.ts: Recognize "opencode" basename (not just
  ".opencode") for direct writes without nesting

Closes #114

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

tests/opencode-writer.test.ts

@@ -59,4 +59,29 @@ describe("writeOpenCodeBundle", () => {
     expect(await exists(path.join(outputRoot, "skills", "skill-one", "SKILL.md"))).toBe(true)
     expect(await exists(path.join(outputRoot, ".opencode"))).toBe(false)
   })
+
+  test("writes directly into ~/.config/opencode style output root", async () => {
+    // Simulates the global install path: ~/.config/opencode
+    const tempRoot = await fs.mkdtemp(path.join(os.tmpdir(), "config-opencode-"))
+    const outputRoot = path.join(tempRoot, ".config", "opencode")
+    const bundle: OpenCodeBundle = {
+      config: { $schema: "https://opencode.ai/config.json" },
+      agents: [{ name: "agent-one", content: "Agent content" }],
+      plugins: [],
+      skillDirs: [
+        {
+          name: "skill-one",
+          sourceDir: path.join(import.meta.dir, "fixtures", "sample-plugin", "skills", "skill-one"),
+        },
+      ],
+    }
+
+    await writeOpenCodeBundle(outputRoot, bundle)
+
+    // Should write directly, not nested under .opencode
+    expect(await exists(path.join(outputRoot, "opencode.json"))).toBe(true)
+    expect(await exists(path.join(outputRoot, "agents", "agent-one.md"))).toBe(true)
+    expect(await exists(path.join(outputRoot, "skills", "skill-one", "SKILL.md"))).toBe(true)
+    expect(await exists(path.join(outputRoot, ".opencode"))).toBe(false)
+  })
 })