[usage] Surface provider-reported token usage instead of only estimating it

[yusiwen]

Refs #174, roadmap entry A2 (tracker: #168).

What changed

• types.Usage (PromptTokens, CompletionTokens, TotalTokens, Add) and
  ChatResponse.Usage *Usage — a pointer, so "the endpoint reported nothing" stays distinguishable
  from "it reported zero" — plus StreamCallbacks.OnUsage.
• agent/provider_openai.go: usage is parsed from the SSE tail, which now reads to data: [DONE]
  instead of breaking at finish_reason — that break is what made the choices-less usage chunk
  unreachable — and from the batch body. A total_tokens the endpoint omits is derived from the two
  halves.
• agent/provider_ollama.go: prompt_eval_count/eval_count from the stream's done line (read
  before the loop breaks) and from the batch body's top level. Counters that are absent stay nil;
  counters that are present and zero are a reported zero.
• agent/agent.go: UsageTotal accumulates every reported call; recordUsage fires OnUsage once
  per reported call, including the max-steps summary call, which builds its own callbacks.
• tui/: UsageMsg carries a report to the UI, and a report replaces the estimate charged to that
  call (callTokens) with the provider's number. sessionTokens stays the single displayed
  counter.
• Tests: agent/usage_test.go (13 functions) and tui/usage_test.go (5).
• Docs: the A2 entry in docs/roadmap.md carries its issue link, and the document now states the
  pickup convention; CODEBASE.md updated for the new fields, events and provider behaviour.

Why

The status bar's tokens: counter was len(text)/4 over streamed output, while the request already
asked every OpenAI-compatible endpoint for real usage (stream_options: {include_usage: true}).
Two separate spots threw that answer away: the choices check skipped the usage-only chunk, and the
loop broke at finish_reason before the chunk was ever read off the socket. Reading past
finish_reason is the only way to reach it.

Rejected alternative: a second counter for real usage. The roadmap asks to extend the existing one,
and two numbers on screen would disagree. The estimate is therefore kept as the fallback for an
endpoint that reports nothing, and a report replaces exactly the share of the counter that call
estimated.

Cost and pricing are deliberately not here: no price table exists in the tree, and the roadmap keeps
that with A3's cumulative-budget work.

Verification

• go test ./... -count=1 → 13 packages ok, 0 failed. go test ./... -count=1 -race → same.

• make staticcheck (v0.8.1) → exit 0. gofmt -l . → no output.

• The new gate was mutation-checked six ways; every mutation failed on an assertion, not a build
  error:

  Mutation	What the test said
  usage read after the choices check	usage chunk after finish_reason was dropped
  break at the finish chunk	usage chunk after finish_reason was dropped
  TUI adds the report without taking back the estimate	tokens: 922, want the reported total 912
  no callTokens reset at the step boundary	callTokens = 10, want 0 at the step boundary
  summary call's usage not recorded	OnUsage fired 1 times, want 2 (step + summary)
  Ollama done-line counters dropped	done-line counters were dropped

• Every new fixture places the usage chunk after the finish_reason chunk, which is the order the
  bug hid behind.

Honest scope

• No live provider call was made from this machine; parsing is pinned by fixtures that mirror the
  documented chunk order (OpenAI's include_usage tail, Ollama's done line).
• Behaviour change: an SSE stream that sends finish_reason and then holds the connection open
  without [DONE] is now read until the provider's own 120 s timeout, where the old code returned at
  finish_reason. The SSE contract ends a stream with [DONE], and a stall before finish_reason
  already blocked for the same bound.
• tokens: now includes prompt tokens, so the number jumps well above the old estimate on the first
  reported call. That is the point of the change rather than a regression, but it is visible.
• Cost/pricing accounting and any cumulative budget or circuit breaker remain open — roadmap A3.
• Usage is not persisted into the session file.