feat(mecatui): add an interactive token-usage breakdown panel

Problem

The mecatui footer exposes compact usage facets such as ↑ input, ↓ output, ⊕ cache write, and cache N%, but the symbols and their scopes are not self-explanatory. The footer also cannot show a useful breakdown across main work, Subagents, Parallel branches, and Team members.

Desired change

Add a /tokens TUI panel that explains and breaks down token usage. Make the footer usage segment clickable so mouse users can discover the panel directly; keep the slash command and the ? help legend so the feature is accessible and discoverable without a mouse.

Panel content

• Explain the footer symbols and their scopes:
  • ctx used/window is current context occupancy, not a cumulative spend total.
  • ↑ and ↓ are session-cumulative input and output.
  • ⊕ is cache-write input, a subset of input rather than an extra total.
  • cache N% is cache-read input divided by total input.
• Show raw usage facets for the main session and a grand total across the main session and all observed delegated work.
• Break usage down by Subagent, Parallel branch, and Team member, including running/completed state and enough identity/goal context to distinguish entries.
• When weighted-cost accounting from #687 exists, show its estimated-cost unit separately from raw tokens and name the active multiplier configuration.

Acceptance criteria

• /tokens opens a dedicated, keyboard-navigable panel from mecatui.
• Clicking the footer usage segment opens the same panel; click hit-testing must not interfere with existing footer actions or narrow-layout behavior.
• The ? overlay names /tokens and includes the symbol legend.
• The panel distinguishes current context occupancy from cumulative usage and explains cache-write subset semantics.
• Raw usage is shown for the main session, each observed delegation child/branch/member, and a grand total.
• Grand totals do not double-count aggregate Team or Parallel totals and their constituent members/branches. Any unavoidable reporting gap is explicit rather than fabricated.
• Usage values come from authoritative relayed server events or a dedicated server projection; the UI does not independently infer provider usage or cache semantics.
• Rendering remains useful in narrow terminals and is covered by focused UI tests.
• Update docs/tui.md and public mecatui documentation.

Context

The current footer implementation is cmd/mecatui/ui/footer.go (renderUsageFacets), and the existing help overlay has the compact usage legend in cmd/mecatui/ui/help.go. The footer currently renders session-cumulative ↑/↓/⊕ totals while the context meter is current occupancy. See docs/tui.md “Footer usage segment” and #687.

Start with cmd/mecatui/ui/footer.go, especially renderUsageFacets, and cmd/mecatui/ui/help.go; read docs/tui.md and issue #687 for the existing usage model. Trace how authoritative usage events reach mecatui before planning the panel and focused UI tests. Done means /tokens, footer click handling, help text, narrow-terminal rendering, correct non-duplicated totals, and documentation updates are covered.