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, 19 insertions, 0 deletions

diff --git a/internal/config/config.go b/internal/config/config.go
index 25187cf..892d3f7 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -45,6 +45,24 @@ func (m LocalModel) UseForElaborate() bool {
 	return *m.PreferForElaborate
 }
 
+// RunnersConfig controls which agent runners are registered. Each field is a
+// *bool so that nil (absent from TOML) means "enabled by default", while an
+// explicit false disables the runner without affecting others.
+//
+// The local runner also requires LocalModel.Endpoint to be set; setting
+// Local = true without an endpoint has no effect.
+type RunnersConfig struct {
+	Claude    *bool `toml:"claude"`
+	Gemini    *bool `toml:"gemini"`
+	Local     *bool `toml:"local"`
+	Container *bool `toml:"container"`
+}
+
+func (r RunnersConfig) ClaudeEnabled() bool    { return r.Claude == nil || *r.Claude }
+func (r RunnersConfig) GeminiEnabled() bool    { return r.Gemini == nil || *r.Gemini }
+func (r RunnersConfig) LocalEnabled() bool     { return r.Local == nil || *r.Local }
+func (r RunnersConfig) ContainerEnabled() bool { return r.Container == nil || *r.Container }
+
 type Config struct {
 	DataDir          string        `toml:"data_dir"`
 	DBPath           string        `toml:"-"`
@@ -68,6 +86,7 @@ type Config struct {
 	VAPIDEmail       string        `toml:"vapid_email"`
 	ClaudeConfigDir  string        `toml:"claude_config_dir"`
 	LocalModel       LocalModel    `toml:"local_model"`
+	Runners          RunnersConfig `toml:"runners"`
 }
 
 func Default() (*Config, error) {