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/app/(authed)/_nav.tsx

@@ -18,6 +18,7 @@ export function Nav({ user }: { user: Session["user"] }) {
 
         <nav className="hidden md:flex items-center gap-1 ml-2">
           <NavLink href="/memories">Memories</NavLink>
+          <NavLink href="/snippets">Snippets</NavLink>
           <NavLink href="/projects">Projects</NavLink>
           <NavLink href="/settings">Settings</NavLink>
         </nav>