feat: add [runners] config section to toggle each runner on/off

Adds RunnersConfig{Claude,Gemini,Local,Container *bool} to Config,
parsed from a [runners] TOML section. Each field uses the *bool pointer
pattern (nil = enabled by default, false = disabled) so existing installs
require no config changes.

serve.go / run.go now gate each runner registration on the corresponding
Enabled() method. The pool's runners map remains the authoritative
routing table — absent entries are already skipped by pickAgent and
fail-fast in getRunner, so no executor changes are needed.

Example:
  [runners]
  gemini = false   # disable cloud gemini runner
  local  = true    # explicit (same as default)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 82 insertions, 0 deletions

diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index e4f1a5d..0d47ca0 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -54,6 +54,88 @@ func TestLoadFile_MissingFile_ReturnsError(t *testing.T) {
 	}
 }
 
+func TestRunnersConfig_DefaultsAllEnabled(t *testing.T) {
+	r := RunnersConfig{}
+	if !r.ClaudeEnabled() {
+		t.Error("claude should be enabled by default (nil)")
+	}
+	if !r.GeminiEnabled() {
+		t.Error("gemini should be enabled by default (nil)")
+	}
+	if !r.LocalEnabled() {
+		t.Error("local should be enabled by default (nil)")
+	}
+	if !r.ContainerEnabled() {
+		t.Error("container should be enabled by default (nil)")
+	}
+}
+
+func TestRunnersConfig_ExplicitFalseDisables(t *testing.T) {
+	f := false
+	r := RunnersConfig{Claude: &f, Gemini: &f, Local: &f, Container: &f}
+	if r.ClaudeEnabled() {
+		t.Error("explicit false should disable claude")
+	}
+	if r.GeminiEnabled() {
+		t.Error("explicit false should disable gemini")
+	}
+	if r.LocalEnabled() {
+		t.Error("explicit false should disable local")
+	}
+	if r.ContainerEnabled() {
+		t.Error("explicit false should disable container")
+	}
+}
+
+func TestRunnersConfig_ExplicitTrueEnables(t *testing.T) {
+	tr := true
+	r := RunnersConfig{Claude: &tr, Gemini: &tr, Local: &tr, Container: &tr}
+	if !r.ClaudeEnabled() || !r.GeminiEnabled() || !r.LocalEnabled() || !r.ContainerEnabled() {
+		t.Error("explicit true should keep all runners enabled")
+	}
+}
+
+func TestRunnersConfig_MixedToggles(t *testing.T) {
+	f, tr := false, true
+	r := RunnersConfig{Claude: &tr, Gemini: &f, Local: &tr}
+	if !r.ClaudeEnabled() {
+		t.Error("claude should be enabled")
+	}
+	if r.GeminiEnabled() {
+		t.Error("gemini should be disabled")
+	}
+	if !r.LocalEnabled() {
+		t.Error("local should be enabled")
+	}
+	if !r.ContainerEnabled() {
+		t.Error("container should be enabled (nil = default)")
+	}
+}
+
+func TestRunnersConfig_TOMLRoundTrip(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.toml")
+	toml := "[runners]\ngemini = false\n"
+	if err := os.WriteFile(path, []byte(toml), 0600); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("HOME", dir)
+
+	cfg, err := LoadFile(path)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if cfg.Runners.GeminiEnabled() {
+		t.Error("gemini should be disabled after TOML load")
+	}
+	if !cfg.Runners.ClaudeEnabled() {
+		t.Error("claude should remain enabled (not in TOML)")
+	}
+	if !cfg.Runners.LocalEnabled() {
+		t.Error("local should remain enabled (not in TOML)")
+	}
+}
+
 func TestLocalModel_UseForElaborate_EmptyEndpoint(t *testing.T) {
 	m := LocalModel{}
 	if m.UseForElaborate() {