refactor(reasoning): unify reasoning extraction across providers

Reasoning surfacing was split across three branches in runner.py plus
two separate streaming buffers (loop hook and runner progress stream),
with three independent display-side gates in the CLI. This collapsed
the policy into one source of truth and fixed two real bugs:

- Structured `reasoning_content` was suppressed whenever the answer was
  streamed, because the runner gated emission on `streamed_content`.
  Providers don't stream `reasoning_content`; it only arrives on the
  final response, so the answer stream and the reasoning channel are
  independent. Added `streamed_reasoning` to `AgentHookContext` to track
  the right bit.
- `channels.showReasoning` was subordinated to `sendProgress`. They are
  orthogonal — turning off progress streaming shouldn't silence
  reasoning. Reworked the CLI gates accordingly.

Single-helper consolidation:

- `extract_reasoning(reasoning_content, thinking_blocks, content)`
  returns `(reasoning_text, cleaned_content)` with a defined fallback
  order: dedicated field → Anthropic thinking_blocks → inline
  `<think>`/`<thought>` tags. Models that expose none of these
  short-circuit to `(None, content)` — zero overhead.
- `IncrementalThinkExtractor` replaces the ad-hoc `emit_incremental_think`
  function and its hand-rolled "emitted cursor" state in both the loop
  hook and the runner progress stream.

Also documented the new `showReasoning` channel option in
docs/configuration.md and noted its independence from sendProgress.

Co-authored-by: Cursor <cursoragent@cursor.com>

nanobot/agent/hook.py

@@ -22,6 +22,7 @@ class AgentHookContext:
     tool_results: list[Any] = field(default_factory=list)
     tool_events: list[dict[str, str]] = field(default_factory=list)
     streamed_content: bool = False
+    streamed_reasoning: bool = False
     final_content: str | None = None
     stop_reason: str | None = None
     error: str | None = None