fix(config): restore sync.go with warning implementations

Commit 356ab92b deleted internal/config/sync.go while adding
cmd/bd/sync_mode.go, but config.go still references the types and
functions from sync.go, causing build failures.

These serve different purposes:
- internal/config/sync.go: viper-based (config.yaml), has warnings
- cmd/bd/sync_mode.go: storage-based (database), no warnings

Both should coexist. This restores sync.go and sync_test.go from
e82e15a8, and fixes type conversion in sync.go:805.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/sync.go

@@ -0,0 +1,140 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+// Sync mode configuration values (from hq-ew1mbr.3)
+// These control how Dolt syncs with JSONL/remotes.
+
+// SyncMode represents the sync mode configuration
+type SyncMode string
+
+const (
+	// SyncModeGitPortable exports JSONL on push, imports on pull (default)
+	SyncModeGitPortable SyncMode = "git-portable"
+	// SyncModeRealtime exports JSONL on every change (legacy behavior)
+	SyncModeRealtime SyncMode = "realtime"
+	// SyncModeDoltNative uses Dolt remote directly (dolthub://, gs://, s3://)
+	SyncModeDoltNative SyncMode = "dolt-native"
+	// SyncModeBeltAndSuspenders uses Dolt remote + JSONL backup
+	SyncModeBeltAndSuspenders SyncMode = "belt-and-suspenders"
+)
+
+// validSyncModes is the set of allowed sync mode values
+var validSyncModes = map[SyncMode]bool{
+	SyncModeGitPortable:       true,
+	SyncModeRealtime:          true,
+	SyncModeDoltNative:        true,
+	SyncModeBeltAndSuspenders: true,
+}
+
+// ConflictStrategy represents the conflict resolution strategy
+type ConflictStrategy string
+
+const (
+	// ConflictStrategyNewest uses last-write-wins (default)
+	ConflictStrategyNewest ConflictStrategy = "newest"
+	// ConflictStrategyOurs prefers local changes
+	ConflictStrategyOurs ConflictStrategy = "ours"
+	// ConflictStrategyTheirs prefers remote changes
+	ConflictStrategyTheirs ConflictStrategy = "theirs"
+	// ConflictStrategyManual requires manual resolution
+	ConflictStrategyManual ConflictStrategy = "manual"
+)
+
+// validConflictStrategies is the set of allowed conflict strategy values
+var validConflictStrategies = map[ConflictStrategy]bool{
+	ConflictStrategyNewest: true,
+	ConflictStrategyOurs:   true,
+	ConflictStrategyTheirs: true,
+	ConflictStrategyManual: true,
+}
+
+// Sovereignty represents the federation sovereignty tier
+type Sovereignty string
+
+const (
+	// SovereigntyT1 is the most open tier (public repos)
+	SovereigntyT1 Sovereignty = "T1"
+	// SovereigntyT2 is organization-level
+	SovereigntyT2 Sovereignty = "T2"
+	// SovereigntyT3 is pseudonymous
+	SovereigntyT3 Sovereignty = "T3"
+	// SovereigntyT4 is anonymous
+	SovereigntyT4 Sovereignty = "T4"
+)
+
+// validSovereigntyTiers is the set of allowed sovereignty values
+var validSovereigntyTiers = map[Sovereignty]bool{
+	SovereigntyT1: true,
+	SovereigntyT2: true,
+	SovereigntyT3: true,
+	SovereigntyT4: true,
+}
+
+// GetSyncMode retrieves the sync mode configuration.
+// Returns the configured mode, or SyncModeGitPortable (default) if not set or invalid.
+// Logs a warning to stderr if an invalid value is configured.
+//
+// Config key: sync.mode
+// Valid values: git-portable, realtime, dolt-native, belt-and-suspenders
+func GetSyncMode() SyncMode {
+	value := GetString("sync.mode")
+	if value == "" {
+		return SyncModeGitPortable // Default
+	}
+
+	mode := SyncMode(strings.ToLower(strings.TrimSpace(value)))
+	if !validSyncModes[mode] {
+		fmt.Fprintf(os.Stderr, "Warning: invalid sync.mode %q in config (valid: git-portable, realtime, dolt-native, belt-and-suspenders), using default 'git-portable'\n", value)
+		return SyncModeGitPortable
+	}
+
+	return mode
+}
+
+// GetConflictStrategy retrieves the conflict resolution strategy configuration.
+// Returns the configured strategy, or ConflictStrategyNewest (default) if not set or invalid.
+// Logs a warning to stderr if an invalid value is configured.
+//
+// Config key: conflict.strategy
+// Valid values: newest, ours, theirs, manual
+func GetConflictStrategy() ConflictStrategy {
+	value := GetString("conflict.strategy")
+	if value == "" {
+		return ConflictStrategyNewest // Default
+	}
+
+	strategy := ConflictStrategy(strings.ToLower(strings.TrimSpace(value)))
+	if !validConflictStrategies[strategy] {
+		fmt.Fprintf(os.Stderr, "Warning: invalid conflict.strategy %q in config (valid: newest, ours, theirs, manual), using default 'newest'\n", value)
+		return ConflictStrategyNewest
+	}
+
+	return strategy
+}
+
+// GetSovereignty retrieves the federation sovereignty tier configuration.
+// Returns the configured tier, or SovereigntyT1 (default) if not set or invalid.
+// Logs a warning to stderr if an invalid value is configured.
+//
+// Config key: federation.sovereignty
+// Valid values: T1, T2, T3, T4
+func GetSovereignty() Sovereignty {
+	value := GetString("federation.sovereignty")
+	if value == "" {
+		return SovereigntyT1 // Default
+	}
+
+	// Normalize to uppercase for comparison (T1, T2, etc.)
+	tier := Sovereignty(strings.ToUpper(strings.TrimSpace(value)))
+	if !validSovereigntyTiers[tier] {
+		fmt.Fprintf(os.Stderr, "Warning: invalid federation.sovereignty %q in config (valid: T1, T2, T3, T4), using default 'T1'\n", value)
+		return SovereigntyT1
+	}
+
+	return tier
+}