## Why

The `GetBoard` API response includes `cardLabels`, `labels`, and `cardMemberships` in its `included` data, but `pcli` only parses `lists` and `cards` — silently discarding the rest. This means there is no way to determine which labels are attached to which cards without making per-card API calls. The kanban sync workflow needs to identify agent-labelled cards from a board listing, and currently must fall back to name-matching because label data is unavailable.

## What Changes

- Expand the `Board` struct in `model/types.go` to include `Labels`, `CardLabels`, and `CardMemberships` fields
- Update `GetBoard` in `client/boards.go` to parse these additional `included` fields from the API response
- Update `ListCardsByBoard` to enrich card output with label names (via `cardLabels` join table + `labels`)

## Capabilities

### New Capabilities

_None — this enhances existing capabilities._

### Modified Capabilities

- `api-client`: `GetBoard` SHALL parse `labels`, `cardLabels`, and `cardMemberships` from the board response `included` data
- `card-operations`: `ListCardsByBoard` (card list --board) SHALL include label names on each card

## Impact

- **Code**: `model/types.go` (Board struct), `client/boards.go` (GetBoard parsing), `client/cards.go` (ListCardsByBoard enrichment)
- **API**: No new API calls — parsing data already returned by `GET /api/boards/:id`
- **Breaking changes**: None — new fields are additive; JSON output gains new fields but existing fields unchanged