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/drizzle/0002_snippets_scope.sql

@@ -0,0 +1,39 @@
+-- Snippets gain scope/project mirroring memories.
+--
+-- Phase 1 created `snippets` as a flat per-user table. To make snippets
+-- behave like memories (user-scope = global, project-scope = tied to a
+-- repo) we add the same three columns: scope, project_id, deleted_at.
+--
+-- Uniqueness of `name` is enforced WITHIN a scope:
+--   - within (user_id) for user-scope rows
+--   - within (user_id, project_id) for project-scope rows
+-- Soft-deleted rows are excluded from uniqueness so a name can be reused
+-- after deletion.
+
+ALTER TABLE "snippets"
+  ADD COLUMN "scope"      memory_scope NOT NULL DEFAULT 'user',
+  ADD COLUMN "project_id" uuid REFERENCES "projects"("id") ON DELETE SET NULL,
+  ADD COLUMN "deleted_at" timestamptz;
+
+-- Scope/project_id consistency mirrors memories_scope_project_chk.
+ALTER TABLE "snippets"
+  ADD CONSTRAINT "snippets_scope_project_chk"
+  CHECK (
+    (scope = 'project' AND project_id IS NOT NULL)
+    OR (scope = 'user' AND project_id IS NULL)
+  );
+
+-- Drop the old global per-user uniqueness; replace with two partial
+-- unique indexes scoped to live (non-deleted) rows.
+DROP INDEX IF EXISTS "snippets_user_name_uq";
+
+CREATE UNIQUE INDEX "snippets_user_name_user_scope_uq"
+  ON "snippets" ("user_id", "name")
+  WHERE scope = 'user' AND deleted_at IS NULL;
+
+CREATE UNIQUE INDEX "snippets_user_project_name_uq"
+  ON "snippets" ("user_id", "project_id", "name")
+  WHERE scope = 'project' AND deleted_at IS NULL;
+
+CREATE INDEX "snippets_user_idx"    ON "snippets" ("user_id");
+CREATE INDEX "snippets_project_idx" ON "snippets" ("project_id");