fix(config): preserve explicit mention_only=false in GroupTriggerConfig

GroupTriggerConfig.MentionOnly was tagged `json:"mention_only,omitempty"`.
Because Go's bool zero value is false, `omitempty` silently drops the field
whenever a user sets it to false. On the next LoadConfig, the missing field
is filled from defaultChannels() (defaults.go:483-484), which hardcodes
`{"mention_only": true}` for channels like LINE and Matrix. LoadConfig then
calls `defer SaveConfig(...)` on any migration branch (config.go:1042,
1083, 1122), writing `true` back to disk.

Net effect: users who explicitly opt out of mention-only mode see their
setting silently revert to true after any upgrade or version migration.
DiscordConfig.MentionOnly already uses the plain `json:"mention_only"` tag
for this reason; this change aligns GroupTriggerConfig with the same
convention.

Add a regression test that fails pre-patch (JSON round-trips to `{}`) and
passes post-patch (explicit false survives marshal/unmarshal).

pkg/config/config.go

@@ -307,7 +307,12 @@ func (d *AgentDefaults) GetModelName() string {
 
 // GroupTriggerConfig controls when the bot responds in group chats.
 type GroupTriggerConfig struct {
-	MentionOnly bool     `json:"mention_only,omitempty"`
+	// MentionOnly must always serialize so an explicit `false` survives the
+	// LoadConfig → SaveConfig roundtrip in migration paths (config.go:1042,
+	// 1083, 1122). With `,omitempty`, Go would omit the bool zero value and
+	// defaultChannels() would re-inject the hardcoded `true` default
+	// (defaults.go:483-484), silently reverting the user's choice.
+	MentionOnly bool     `json:"mention_only"`
 	Prefixes    []string `json:"prefixes,omitempty"`
 }
 

pkg/config/group_trigger_mention_only_test.go

@@ -0,0 +1,43 @@
+package config
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestGroupTriggerConfig_MentionOnly_False_Roundtrips documents the regression
+// where an explicit `mention_only: false` is silently lost on a JSON roundtrip
+// because the struct tag `json:"mention_only,omitempty"` causes Go to omit the
+// field whenever its value equals the bool zero value (false).
+//
+// Chain of damage in production:
+//   1. User edits config.json and sets "mention_only": false.
+//   2. Gateway loads config → LoadConfig triggers defer SaveConfig on any
+//      migration path (config.go:1042/1083/1122).
+//   3. SaveConfig re-marshals the struct. `,omitempty` drops the false field.
+//   4. The next load merges with defaultChannels() (defaults.go:483-484),
+//      which hardcodes `{"mention_only": true}` for the "line" channel.
+//   5. User's explicit false silently becomes true.
+//
+// The fix is to remove `,omitempty` from the tag so false always serializes,
+// matching the pattern already used by DiscordConfig.MentionOnly (no omitempty).
+func TestGroupTriggerConfig_MentionOnly_False_Roundtrips(t *testing.T) {
+	original := GroupTriggerConfig{MentionOnly: false}
+
+	data, err := json.Marshal(original)
+	require.NoError(t, err)
+
+	// The bug: with `,omitempty`, the false field is omitted entirely, so the
+	// JSON becomes "{}" and the user's explicit choice is indistinguishable
+	// from "never set". A caller that fills in defaults for missing fields
+	// will then replace false with true.
+	assert.Contains(t, string(data), `"mention_only":false`,
+		"explicit false must serialize — otherwise defaults logic overwrites it; see defaults.go:483-484")
+
+	var decoded GroupTriggerConfig
+	require.NoError(t, json.Unmarshal(data, &decoded))
+	assert.False(t, decoded.MentionOnly, "mention_only should survive roundtrip as false")
+}