feat(agent/agentcontextconfig): discover skills from ~/.coder/skills (#25271)

The default skills lookup only scanned the project-relative
.agents/skills directory, so personal skills had to be repeated
per project or wired in via CODER_AGENT_EXP_SKILLS_DIRS. Now the
default is the comma-separated list ~/.coder/skills,.agents/skills,
which lets discoverSkills's existing first-occurrence-wins policy
prefer home-scoped skills over project ones with the same name.

The change is additive when ~/.coder/skills is absent
(missing directories are silently skipped in discoverSkills) and
unaffects users who set the env var explicitly.

Closes CODAGT-403

agent/agentcontextconfig/api.go

@@ -60,10 +60,14 @@ var skillNamePattern = regexp.MustCompile(
 
 // Default values for agent-internal configuration. These are
 // used when the corresponding env vars are unset.
+//
+// DefaultSkillsDir is a comma-separated list so home-scoped
+// skills override project-scoped ones with the same name
+// (discoverSkills picks the first occurrence per skill name).
 const (
 	DefaultInstructionsDir  = "~/.coder"
 	DefaultInstructionsFile = "AGENTS.md"
-	DefaultSkillsDir        = ".agents/skills"
+	DefaultSkillsDir        = "~/.coder/skills,.agents/skills"
 	DefaultSkillMetaFile    = "SKILL.md"
 	DefaultMCPConfigFile    = ".mcp.json"
 )

agent/agentcontextconfig/api_test.go

@@ -461,6 +461,37 @@ func TestResolve(t *testing.T) {
 		require.Len(t, skillParts, 1)
 		require.Equal(t, "from skills1", skillParts[0].SkillDescription)
 	})
+
+	//nolint:paralleltest // Uses t.Setenv to mutate HOME.
+	t.Run("DefaultDiscoversHomeAndProjectSkillsHomeWins", func(t *testing.T) {
+		fakeHome := t.TempDir()
+		t.Setenv("HOME", fakeHome)
+		t.Setenv("USERPROFILE", fakeHome)
+		workDir := t.TempDir()
+
+		homeSkills := filepath.Join(fakeHome, ".coder", "skills")
+		writeSkillMetaFileInRoot(t, homeSkills, "home-only", "home only")
+		writeSkillMetaFileInRoot(t, homeSkills, "shared", "from home")
+		writeSkillMetaFile(t, workDir, "project-only", "project only")
+		writeSkillMetaFile(t, workDir, "shared", "from project")
+
+		// Construct the Config directly with the package defaults
+		// to verify the default skills list (and only the defaults).
+		cfg, _ := agentcontextconfig.Resolve(workDir, agentcontextconfig.Config{
+			SkillsDirs:    agentcontextconfig.DefaultSkillsDir,
+			SkillMetaFile: agentcontextconfig.DefaultSkillMetaFile,
+		})
+
+		got := map[string]string{}
+		for _, p := range filterParts(cfg.Parts, codersdk.ChatMessagePartTypeSkill) {
+			got[p.SkillName] = p.SkillDescription
+		}
+		require.Equal(t, map[string]string{
+			"home-only":    "home only",
+			"project-only": "project only",
+			"shared":       "from home",
+		}, got)
+	})
 }
 
 func TestNewAPI_LazyDirectory(t *testing.T) {