Completes Phase 2 of the NIP-AM local agent usage feature. tauriArchive
decodes the backend's persistedAgentMetrics count defensively (missing
field -> 0, per A5/M2's backward-compatible wire contract) and exposes
an onAgentMetricsChanged notifier, fired only when a subscription
mutation touches kind 44200 specifically.

ArchiveSyncManager routes both its idle/size-triggered and destroy-time
flushes through one sendBatch helper that notifies usage listeners
only when the backend reports persistedAgentMetrics > 0 — rejections,
duplicate-only batches, and non-metric successes never notify, keeping
the refresh signal backend-authoritative.

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

Surfaces per-agent NIP-AM turn-metric usage (tokens, cost, model
breakdown, 7d/30d bucket series) in the desktop app, reading from the
Phase 2 get_agent_usage_series archive command:

- agent-usage/lib/agentUsage.ts: pure client-side helpers -- bucket
  boundary construction, bigint-safe token formatting, and the ranked
  overview-row projection shared by both UI surfaces.
- agent-usage/hooks.ts: react-query wrapper around
  get_agent_usage_series with the app's standard retry:1 default.
- agent-usage/ui/AgentUsageSection.tsx: ranked per-agent usage rows on
  the Agents overview, with a 7d/30d window switch, retry-on-error,
  and a collection-off banner (with or without retained-data coverage
  copy) linking to Local Archive settings.
- agent-usage/ui/AgentUsageFocusedView.tsx: per-agent drill-down
  (bucketed series + model breakdown) reached via an overview row or
  the profile panel's Info-tab usage ingress row; wired into
  AgentsView, UserProfilePanel, and ProfilePanelContext.
- Partial badge on rows whose total is a known lower bound
  (has_unknown_usage), surfaced via the accounting engine's per-field
  completeness contract.

Extends the Rust archive test suite (agent_usage_tests.rs) to close
gaps in the pure accounting engine: the f64 cost ladder had no direct
outcome.cost assertions, assign_bucket_index's start-inclusive/
end-exclusive edges were only exercised indirectly, validate_request's
chrono finite-range rejection was unexercised, and the A2 ranking
tiebreak (pubkey ascending on an exact totalTokens tie) had no test
distinguishing it from insertion order.

tests/e2e/agent-usage.spec.ts (wired into playwright.config.ts's smoke
project) exercises both UI surfaces against a mocked
get_agent_usage_series: ranked rows, window switching, click-through
to the focused view from both the overview row and the profile
panel's ingress row, retry recovery from a query error, the
collection-off banner and its settings deep link, and the Partial
badge.

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>
Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>