Fixed the Gitea build break caused by `go:embed` requiring `internal/app/ui/dist` to exist in a clean checkout.

Changes made:
- Added `internal/app/ui/dist/placeholder.txt` so the embedded UI directory is always present in source control.
- Updated `internal/mcpserver/server_test.go` to derive expected tool names from `BuildToolCatalog()` instead of a stale hard-coded list.
- Removed stale maintenance tool entries from `internal/mcpserver/server.go` because those tools are not currently registered.

Verification:
- `env GOCACHE=/tmp/go-build go test ./internal/mcpserver -run TestNewListsAllRegisteredTools -v` passes.
- A broader `go test ./internal/app ./cmd/amcs-server` compile check was started, but it did not finish before this log entry was written.

Migration follow-up:
- Fixed `migrations/020_generated_schema.sql` after PostgreSQL failed with `operator does not exist: name[] = text[]`.
- Root cause: `pg_attribute.attname` is type `name`, so `ARRAY(SELECT a.attname ...)` produced `name[]`, which was compared against `text[]` literals.
- Updated each repeated primary-key introspection block to use `SELECT a.attname::text`, keeping the existing `ARRAY[]::text[]` and `ARRAY['id']` / `ARRAY['persona_id']` comparisons valid.