feat(snippets): named reusable artifacts with scope mirroring memories

Snippets are exact-name-keyed templates (PR formats, checklists, style
rules) — prescriptive artifacts the user wants applied consistently.
Distinct from memories, which are searched descriptive prose.

- Migration 0002 adds scope/project_id/deleted_at to snippets, partial
  unique indexes per scope, and a scope/project CHECK constraint
  mirroring memories_scope_project_chk.
- New @shared-memory/schemas: SnippetName/Put/Get/List/Delete inputs
  with shared scope-project refinement.
- lib/snippets.ts: get/put/list/softDelete helpers used by both the
  Web Server Actions and the MCP tool handlers.
- Four new MCP tools (snippet.put/get/list/delete) with directive
  descriptions contrasting against memory.* (exact-name lookup vs
  search; templates vs facts).
- /snippets pages: list, new, [name] with edit + delete and a
  scope-picker when a name lives in multiple scopes.
- Nav: Snippets link between Memories and Projects.

Design note: when a name exists in both user and project scope,
snippet.get without an explicit scope prefers project (if project key
given) then falls back to user. The detail page shows a picker when
the name is ambiguous and no scope query param was supplied.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/lib/db/schema.ts

@@ -114,15 +114,22 @@ export const snippets = pgTable(
     userId: uuid("user_id")
       .notNull()
       .references(() => users.id, { onDelete: "cascade" }),
+    // NULL when scope = 'user' (global to the user). Mirrors `memories`.
+    projectId: uuid("project_id").references(() => projects.id, { onDelete: "set null" }),
+    scope: memoryScope("scope").notNull().default("user"),
     name: varchar("name", { length: 200 }).notNull(),
     body: text("body").notNull(),
     description: text("description"),
     tags: textArray("tags").notNull().default([]),
     createdAt: timestamp("created_at", { withTimezone: true }).notNull().defaultNow(),
     updatedAt: timestamp("updated_at", { withTimezone: true }).notNull().defaultNow(),
+    deletedAt: timestamp("deleted_at", { withTimezone: true }),
   },
   (t) => ({
-    uniqueUserName: uniqueIndex("snippets_user_name_uq").on(t.userId, t.name),
+    userIdx: index("snippets_user_idx").on(t.userId),
+    projectIdx: index("snippets_project_idx").on(t.projectId),
+    // Partial unique indexes (one per scope, live rows only) are declared
+    // in the SQL migration since drizzle-kit doesn't model partial indexes.
   }),
 );