The terminal's Usage screen opens the fleet graph with `f`: your repositories' spend and runs per hour over 24 h and per day over 14 d #8048

Intent: to be ultra aware of the forge's state, see where the spend and runs went over time from the terminal, as /fleet/graph shows on the web.

Done looks like:

• [ ] A new src/tui/screens/fleetgraph.ts reads fleetGraphFor / readFleetGraph from src/actions/fleet-graph.ts (the same data the page, CSV and API answer from). It draws one sparkline row per series (up to FLEET_GRAPH_MAX_SERIES) for runs and for spend, with each row's total in dollars and runs at the right. w toggles 24h ↔ 14d.
• [ ] It is opened with f from src/tui/screens/usage.ts, and the binding is listed in that screen's keys.
• [ ] Someone who may not see spend gets what the page gives them, never more: the action's visibility rules decide, not the screen.
• [ ] test/tui-fleetgraph.test.ts: with usage ledger lines seeded in two repositories, the screen names both and their totals match readFleetGraph. w switches the window. An empty ledger reads "Nothing spent in this window."

Files: src/tui/screens/fleetgraph.ts (new), src/tui/screens/usage.ts, test/tui-fleetgraph.test.ts. Risks: narrow terminals. Truncate names and keep the graph at least 10 columns, using the existing truncate/split helpers.