feat: add Google Antigravity as a skill host (#50)

Antigravity reads global skills from ~/.gemini/config/skills (and
project skills from <workspace>/.agents/skills). Upstream praxis wrote
to none of these dirs, so no skills ever surfaced in Antigravity —
unlike Codex, which is covered by the existing ~/.agents/skills adapter.

Add a fourth harness that sources skills into ~/.gemini/config/skills,
detected via the `agy` / `antigravity-ide` binaries or the
~/.gemini/antigravity-ide app-data dir. Detection deliberately avoids a
bare ~/.gemini (Gemini CLI's signal, which Antigravity also creates) so
the two hosts are never conflated. Agents are gated off (skills only),
same posture as Codex, pending verification of Antigravity's subagent
loader.

Verified end to end against a real Antigravity install: refresh-skills
lands 29 skill packages and the Antigravity IDE/agent enumerates them;
with the change reverted, ~/.gemini/config/skills stays empty and the
agent reports no skills.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: SatyendraGollaFacets

internal/harness/harness.go

@@ -1,8 +1,14 @@
 // Package harness detects which AI host CLIs/IDEs are present locally and
-// reports where each one looks for skill files. The 3 harnesses listed
+// reports where each one looks for skill files. The harnesses listed
 // here all support the Agent Skills open standard at user scope, so
 // `praxis skill install` writes the same SKILL.md to each detected one.
 //
+// Google Antigravity and Gemini CLI both root their config under
+// ~/.gemini, but at different skill dirs: Gemini CLI reads ~/.gemini/skills
+// while Antigravity reads ~/.gemini/config/skills (its config root, marked
+// by ~/.gemini/config/.migrated). They are detected by distinct signals so
+// a bare ~/.gemini does not misattribute one for the other.
+//
 // Cursor is intentionally NOT included: it has no user-scope skills
 // directory (only project-scope under .cursor/skills/), so it requires
 // per-repo handling and is deferred to a later release.
@@ -18,7 +24,7 @@ import (
 
 // Harness is one supported AI host.
 type Harness struct {
-	Name        string // canonical id: "claude-code", "codex", "gemini-cli"
+	Name        string // canonical id: "claude-code", "codex", "gemini-cli", "antigravity"
 	DisplayName string // human label
 	Detected    bool   // present on this machine
 	BinaryPath  string // resolved binary path if found in $PATH
@@ -33,12 +39,13 @@ func All() []Harness {
 		detectClaudeCode(home),
 		detectCodex(home),
 		detectGeminiCLI(home),
+		detectAntigravity(home),
 	}
 }
 
 // Detected returns only the harnesses present on this machine.
 func Detected() []Harness {
-	out := make([]Harness, 0, 3)
+	out := make([]Harness, 0, 4)
 	for _, h := range All() {
 		if h.Detected {
 			out = append(out, h)
@@ -114,6 +121,43 @@ func detectGeminiCLI(home string) Harness {
 	return h
 }
 
+func detectAntigravity(home string) Harness {
+	h := Harness{
+		Name:        "antigravity",
+		DisplayName: "Google Antigravity",
+		// Antigravity's canonical config root is ~/.gemini/config (it writes
+		// a migration marker at ~/.gemini/config/.migrated on first run).
+		// Global user-scope skills live under that config dir, and per the
+		// official docs each is a folder containing SKILL.md.
+		SkillDir: filepath.Join(home, ".gemini", "config", "skills"),
+		// AgentDir is set for struct consistency but unused: Antigravity
+		// agent install is gated off (see agentinstall.supportsAgentInstall),
+		// pending verification of its subagent loader path — same posture as
+		// Codex. Only the skill dir above is written today.
+		AgentDir: filepath.Join(home, ".gemini", "config", "agents"),
+	}
+	// Detect via the Antigravity binaries (CLI `agy`, IDE `antigravity-ide`)
+	// or its app-data dirs. Deliberately NOT keyed off a bare ~/.gemini —
+	// that is shared with Gemini CLI; the antigravity-ide subdir / app-data
+	// dir disambiguates so the two hosts are never conflated.
+	for _, bin := range []string{"agy", "antigravity-ide"} {
+		if p, err := exec.LookPath(bin); err == nil {
+			h.Detected = true
+			h.BinaryPath = p
+			break
+		}
+	}
+	for _, dir := range []string{
+		filepath.Join(home, ".gemini", "antigravity-ide"),
+		filepath.Join(home, ".antigravity-ide"),
+	} {
+		if _, err := os.Stat(dir); err == nil {
+			h.Detected = true
+		}
+	}
+	return h
+}
+
 // ProjectScoped returns a copy of h whose SkillDir and AgentDir are
 // rebased from the user's home directory onto projectDir. For example a
 // Claude Code SkillDir of ~/.claude/skills becomes

internal/harness/harness_test.go

@@ -18,13 +18,13 @@ func withIsolatedHome(t *testing.T) string {
 	return home
 }
 
-func TestAll_ReturnsThreeHarnesses(t *testing.T) {
+func TestAll_ReturnsAllHarnesses(t *testing.T) {
 	withIsolatedHome(t)
 	got := All()
-	if len(got) != 3 {
-		t.Errorf("len(All()) = %d, want 3", len(got))
+	if len(got) != 4 {
+		t.Errorf("len(All()) = %d, want 4", len(got))
 	}
-	wantNames := map[string]bool{"claude-code": true, "codex": true, "gemini-cli": true}
+	wantNames := map[string]bool{"claude-code": true, "codex": true, "gemini-cli": true, "antigravity": true}
 	for _, h := range got {
 		if !wantNames[h.Name] {
 			t.Errorf("unexpected harness %q in All()", h.Name)
@@ -101,6 +101,32 @@ func TestDetect_GeminiCLI_HomeDir(t *testing.T) {
 	}
 }
 
+func TestDetect_Antigravity_AppDataDir(t *testing.T) {
+	home := withIsolatedHome(t)
+	// The antigravity-ide app-data subdir under ~/.gemini is the distinct
+	// signal that disambiguates Antigravity from Gemini CLI.
+	if err := os.MkdirAll(filepath.Join(home, ".gemini", "antigravity-ide"), 0700); err != nil {
+		t.Fatal(err)
+	}
+	h, _ := ByName("antigravity")
+	if !h.Detected {
+		t.Errorf("Antigravity not detected when ~/.gemini/antigravity-ide exists: %+v", h)
+	}
+}
+
+func TestDetect_Antigravity_NotOnBareGemini(t *testing.T) {
+	home := withIsolatedHome(t)
+	// A bare ~/.gemini (Gemini CLI's signal) must NOT trip Antigravity —
+	// the two share the ~/.gemini root and must stay disambiguated.
+	if err := os.MkdirAll(filepath.Join(home, ".gemini"), 0700); err != nil {
+		t.Fatal(err)
+	}
+	h, _ := ByName("antigravity")
+	if h.Detected {
+		t.Errorf("Antigravity must not be detected on a bare ~/.gemini: %+v", h)
+	}
+}
+
 func TestSkillDir_PerHarness(t *testing.T) {
 	home := withIsolatedHome(t)
 	tests := []struct {
@@ -110,6 +136,7 @@ func TestSkillDir_PerHarness(t *testing.T) {
 		{"claude-code", filepath.Join(home, ".claude", "skills")},
 		{"codex", filepath.Join(home, ".agents", "skills")},
 		{"gemini-cli", filepath.Join(home, ".gemini", "skills")},
+		{"antigravity", filepath.Join(home, ".gemini", "config", "skills")},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -249,6 +276,7 @@ func TestAllHarnessesHaveAgentDir(t *testing.T) {
 		"claude-code": filepath.Join(home, ".claude", "agents"),
 		"codex":       filepath.Join(home, ".codex", "agents"),
 		"gemini-cli":  filepath.Join(home, ".gemini", "agents"),
+		"antigravity": filepath.Join(home, ".gemini", "config", "agents"),
 	}
 	for _, h := range All() {
 		got, ok := want[h.Name]