The terminal's Usage screen, opened from the dashboard with no repository, shows the forge's spend today, in the last 24 h and each subscription's headroom #8308

Third part of #7656: the same answer through the terminal's door.

Done when:

• [ ] Opening Usage from the forge's palette with no repository draws three lines: today, last 24 h, and each subscription's headroom as a percent, from the forge-wide usage function
• [ ] The per-repository Usage screen is unchanged
• [ ] A TUI test renders the screen for a caller with two repositories and proves the three lines. It fails without the change

Files: src/tui/screens/usage.ts, src/tui/forge.ts, test/tui-usage-forge.test.ts Risk: follow AGENTS.md's rule for where the palette entry lives.