feat(tui): show execute threshold in sidebar + status header

Sidebar header now displays the per-model execute-threshold alongside
the current usage percentage, in a left/right split layout:

  47.5% / 65%                         475K / 1.0M

The left half tells the user how close they are to the next compaction
trigger; the right half stays the absolute "tokens used / context
window" reading.

Status dialog header gets the same shape for visual consistency (was
previously "Context  47.5% percent dot 475K / 1.0M tokens" on one
right-aligned line).

Implementation:

  - `executeThreshold: number` added to `SidebarSnapshot` so the sidebar
    can render it without a second RPC round-trip
  - `buildSidebarSnapshot` now accepts an optional `config` arg and
    resolves the threshold per-model via the same
    `resolveExecuteThresholdDetail` flow used by `buildStatusDetail`
  - Both RPC handler call sites forward the raw config so the sidebar
    and dialog agree on the resolved value
  - Sticky cache and client EMPTY_SNAPSHOT updated for the new field;
    the cache merger inherits `executeThreshold` from the fresh
    snapshot via `...fresh` so model switches surface immediately

Tests: 1479 pass / 0 fail. Lint, typecheck, build clean.