refactor(store,tools): migrate IDs from UUID to bigserial int64

All internal entity lookups now use bigserial primary keys (int64) while
GUIDs are retained for external/public identification. Updated store
functions (TouchProject, UpdateThoughtMetadata, AddThoughtAttachment) to
query by id instead of guid, added GetThoughtByID, changed semanticSearch
and all tool helpers to use *int64 project IDs, and updated retry/backfill
workers to use int64 thought IDs throughout.

internal/types/extensions.go

@@ -217,7 +217,8 @@ type ContactHistory struct {
 // Agent Skills & Guardrails
 
 type AgentSkill struct {
-	ID          uuid.UUID `json:"id"`
+	ID          int64     `json:"id"`
+	GUID        uuid.UUID `json:"guid"`
 	Name        string    `json:"name"`
 	Description string    `json:"description,omitempty"`
 	Content     string    `json:"content"`
@@ -227,7 +228,8 @@ type AgentSkill struct {
 }
 
 type AgentGuardrail struct {
-	ID          uuid.UUID `json:"id"`
+	ID          int64     `json:"id"`
+	GUID        uuid.UUID `json:"guid"`
 	Name        string    `json:"name"`
 	Description string    `json:"description,omitempty"`
 	Content     string    `json:"content"`
@@ -245,12 +247,13 @@ type ChatMessage struct {
 }
 
 type ChatHistory struct {
-	ID        uuid.UUID      `json:"id"`
+	ID        int64          `json:"id"`
+	GUID      uuid.UUID      `json:"guid"`
 	SessionID string         `json:"session_id"`
 	Title     string         `json:"title,omitempty"`
 	Channel   string         `json:"channel,omitempty"`
 	AgentID   string         `json:"agent_id,omitempty"`
-	ProjectID *uuid.UUID     `json:"project_id,omitempty"`
+	ProjectID *int64         `json:"project_id,omitempty"`
 	Messages  []ChatMessage  `json:"messages"`
 	Summary   string         `json:"summary,omitempty"`
 	Metadata  map[string]any `json:"metadata"`