feat(channels): add Facebook Messenger and Pancake channel integrations (#731)

Add two new channel implementations for Facebook Fanpage (comment + Messenger
auto-reply, first inbox DM) and Pancake/pages.fm (multi-platform inbox via
Facebook, Zalo, Instagram, TikTok, WhatsApp, LINE).

Key features:
- Facebook: comment auto-reply, Messenger auto-reply, first inbox DM,
  HMAC-SHA256 webhook verification, multi-page webhook routing
- Pancake: multi-platform inbox, outbound echo dedup with HTML normalization,
  race-condition-safe echo fingerprinting, platform-aware formatting
- Bootstrap skip: pre-fill USER.md from channel metadata (Pancake)
- SanitizeDisplayName across all channels (defense in depth)

Code audit fixes:
- Fix truncateForTikTok byte→rune slicing (UTF-8 corruption)
- Fix empty message.ID shared dedup slot (silent message loss)
- Fix DisplayName markdown injection in buildPrefilledUser
- Consolidate duplicate ChannelMeta type (agent→bootstrap)
- Compile-time interface assertions, alphabetical type constants
- Per-message logs demoted to slog.Debug, errors.As for wrapped errors
- UI: alphabetical channel ordering, complete config schemas
- Remove deprecated WhatsApp bridge_url, fix nested error parsing

internal/agent/loop_utils.go

@@ -64,10 +64,23 @@ func (l *Loop) shouldShareKnowledgeGraph() bool {
 	return l.workspaceSharing != nil && l.workspaceSharing.ShareKnowledgeGraph
 }
 
+// buildChannelMeta extracts channel metadata from RunRequest for bootstrap decisions.
+// Returns nil when channel type is unknown (preserves normal bootstrap flow).
+func (l *Loop) buildChannelMeta(req *RunRequest) *bootstrap.ChannelMeta {
+	if req == nil || req.ChannelType == "" {
+		return nil
+	}
+	return &bootstrap.ChannelMeta{
+		ChannelType:     req.ChannelType,
+		DisplayName:     req.SenderName,
+		DefaultTimezone: l.defaultTimezone,
+	}
+}
+
 // getOrCreateUserSetup returns the cached userSetup for a user, creating it on first call.
 // On first call: seeds context files (non-team) and resolves workspace from user profile.
 // On subsequent calls: returns cached setup immediately (no DB calls).
-func (l *Loop) getOrCreateUserSetup(ctx context.Context, userID, channel string, isTeamSession bool) *userSetup {
+func (l *Loop) getOrCreateUserSetup(ctx context.Context, userID, channel string, isTeamSession bool, channelMeta *bootstrap.ChannelMeta) *userSetup {
 	if userID == "" {
 		return &userSetup{workspace: l.workspace}
 	}
@@ -92,7 +105,7 @@ func (l *Loop) getOrCreateUserSetup(ctx context.Context, userID, channel string,
 			}
 			// Step 2: Seed context files (must run before buildMessages reads them).
 			// Passes isNew so SeedUserFiles knows whether to skip existing files.
-			if err := l.seedUserFiles(ctx, l.agentUUID, userID, l.agentType, isNew); err != nil {
+			if err := l.seedUserFiles(ctx, l.agentUUID, userID, l.agentType, isNew, channelMeta); err != nil {
 				slog.Warn("failed to seed user context files", "error", err)
 				// Seeding failed (e.g. SQLITE_BUSY after retries). Inject
 				// embedded bootstrap templates in-memory so the first turn