fix(execenv): write Copilot skills to .github/skills/ for native discovery (#1270)

GitHub Copilot CLI scans project-level skills from .github/skills/<name>/SKILL.md
(per the official cli-config-dir-reference docs), not from .agent_context/skills/.
Previously, skills injected for the copilot provider were placed under
.agent_context/skills/ and only referenced by name in AGENTS.md, meaning
Copilot would not actually pick them up.

- resolveSkillsDir: add a dedicated copilot case writing to .github/skills/
- Update doc comments in context.go and runtime_config.go
- Add TestWriteContextFilesCopilotNativeSkills covering the new path and
  ensuring .agent_context/skills/ is not created for copilot

Co-authored-by: Devv <devv@Devvs-Mac-mini.local>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

server/internal/daemon/execenv/context.go

@@ -13,7 +13,7 @@ import (
 //
 // Claude:   skills → {workDir}/.claude/skills/{name}/SKILL.md  (native discovery)
 // Codex:    skills → handled separately in Prepare via codex-home
-// Copilot:  skills → {workDir}/.agent_context/skills/{name}/SKILL.md  (via AGENTS.md references)
+// Copilot:  skills → {workDir}/.github/skills/{name}/SKILL.md  (native project-level discovery)
 // OpenCode: skills → {workDir}/.config/opencode/skills/{name}/SKILL.md  (native discovery)
 // Pi:       skills → {workDir}/.pi/agent/skills/{name}/SKILL.md  (native discovery)
 // Cursor:   skills → {workDir}/.cursor/skills/{name}/SKILL.md  (native discovery)
@@ -54,6 +54,12 @@ func resolveSkillsDir(workDir, provider string) (string, error) {
 	case "claude":
 		// Claude Code natively discovers skills from .claude/skills/ in the workdir.
 		skillsDir = filepath.Join(workDir, ".claude", "skills")
+	case "copilot":
+		// GitHub Copilot CLI natively discovers project-level skills from
+		// .github/skills/<name>/SKILL.md (takes precedence over user-level
+		// skills in ~/.copilot/skills/).
+		// See: https://docs.github.com/en/copilot/reference/copilot-cli-reference/cli-config-dir-reference
+		skillsDir = filepath.Join(workDir, ".github", "skills")
 	case "opencode":
 		// OpenCode natively discovers skills from .config/opencode/skills/ in the workdir.
 		skillsDir = filepath.Join(workDir, ".config", "opencode", "skills")

server/internal/daemon/execenv/execenv_test.go

@@ -479,6 +479,56 @@ func TestInjectRuntimeConfigNoSkills(t *testing.T) {
 	}
 }
 
+func TestWriteContextFilesCopilotNativeSkills(t *testing.T) {
+	t.Parallel()
+	dir := t.TempDir()
+
+	ctx := TaskContextForEnv{
+		IssueID: "copilot-skill-test",
+		AgentSkills: []SkillContextForEnv{
+			{
+				Name:    "Go Conventions",
+				Content: "Follow Go conventions.",
+				Files: []SkillFileContextForEnv{
+					{Path: "templates/example.go", Content: "package main"},
+				},
+			},
+		},
+	}
+
+	if err := writeContextFiles(dir, "copilot", ctx); err != nil {
+		t.Fatalf("writeContextFiles failed: %v", err)
+	}
+
+	// Copilot CLI natively discovers project-level skills from .github/skills/.
+	skillMd, err := os.ReadFile(filepath.Join(dir, ".github", "skills", "go-conventions", "SKILL.md"))
+	if err != nil {
+		t.Fatalf("failed to read .github/skills/go-conventions/SKILL.md: %v", err)
+	}
+	if !strings.Contains(string(skillMd), "Follow Go conventions.") {
+		t.Error("SKILL.md missing content")
+	}
+
+	// Supporting files should also be under .github/skills/.
+	supportFile, err := os.ReadFile(filepath.Join(dir, ".github", "skills", "go-conventions", "templates", "example.go"))
+	if err != nil {
+		t.Fatalf("failed to read supporting file: %v", err)
+	}
+	if string(supportFile) != "package main" {
+		t.Errorf("supporting file content = %q, want %q", string(supportFile), "package main")
+	}
+
+	// .agent_context/skills/ should NOT exist for Copilot.
+	if _, err := os.Stat(filepath.Join(dir, ".agent_context", "skills")); !os.IsNotExist(err) {
+		t.Error("expected .agent_context/skills/ to NOT exist for Copilot provider")
+	}
+
+	// issue_context.md should still be in .agent_context/.
+	if _, err := os.Stat(filepath.Join(dir, ".agent_context", "issue_context.md")); os.IsNotExist(err) {
+		t.Error("expected .agent_context/issue_context.md to exist")
+	}
+}
+
 func TestWriteContextFilesOpencodeNativeSkills(t *testing.T) {
 	t.Parallel()
 	dir := t.TempDir()

server/internal/daemon/execenv/runtime_config.go

@@ -12,7 +12,7 @@ import (
 //
 // For Claude:   writes {workDir}/CLAUDE.md  (skills discovered natively from .claude/skills/)
 // For Codex:    writes {workDir}/AGENTS.md  (skills discovered natively via CODEX_HOME)
-// For Copilot:  writes {workDir}/AGENTS.md  (Copilot CLI natively reads AGENTS.md)
+// For Copilot:  writes {workDir}/AGENTS.md  (skills discovered natively from .github/skills/)
 // For OpenCode: writes {workDir}/AGENTS.md  (skills discovered natively from .config/opencode/skills/)
 // For OpenClaw: writes {workDir}/AGENTS.md  (skills discovered natively from .openclaw/skills/)
 // For Gemini:   writes {workDir}/GEMINI.md  (discovered natively by the Gemini CLI)