(MOT-4251) feat(console): add session usage metrics

[ytallo]

Summary

• Surface provider-reported input, output, cache, reasoning, and cost metrics in a session metrics dialog.
• Aggregate usage by turn and across the spawned session tree, while preserving the distinction between zero and fields a provider did not report.
• Add persisted per-turn usage chips, Storybook coverage, unit tests, and Console E2E scenarios.

Why

The Console currently shows only an estimated context gauge, so developers cannot inspect the exact usage and cost reported by providers or understand how work is distributed across turns and sub-agents. These metrics make session consumption visible without presenting estimates as exact values.

Impact

Developers can open metrics from the Console header or context gauge, compare exact, counted, and estimated values, inspect per-turn model calls, review whole-tree and trace totals, and optionally hide transcript usage chips.

Validation

• pnpm --dir console/web exec biome check <changed files>
• pnpm --dir console/web typecheck
• pnpm --dir console/web typecheck:e2e
• pnpm --dir console/web exec vitest run src/lib/session-usage.test.ts src/lib/sessions/entry-mapper.test.ts — 70 tests passed
• pnpm --dir console/web build
• pnpm --dir console/web exec playwright test e2e/ui-metrics.spec.ts against the pinned integration stack — 2 tests passed

Fixes MOT-4251

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• Commit unit tests in branch feat/console-session-metrics

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[github-actions]

skill-check — worker

0 verified, 49 skipped (no docs/).

Layer	Result
structure	✓
vale	✓
ai	✓
render	✓

Four for four. Nicely done.