Extract shared chatrun package to eliminate chat-runner DRY violations

The CLI, LSP server and tmux code-action tool each carried near-identical
chat-running logic: invoking the LLM (streaming-aware), collecting the
response, and accounting sent/received bytes into the stats package.

Introduce internal/chatrun with:
- Invoke: streaming-aware LLM call that collects the full response and
  optionally mirrors chunks to a writer (nil writer = collect only).
- SentBytes / Account: shared byte counting and stats.Update.

Wire all three surfaces to it:
- hexaicli: runChatRequest delegates to chatrun.Invoke; summarizeChatRun
  uses chatrun.Account. Removed the duplicated streaming/simple helpers.
- hexaiaction: runOnce uses chatrun.Invoke + chatrun.Account, keeping the
  tmux status update local.
- lsp: chatWithStats and the completion path use chatrun.SentBytes/Invoke;
  extracted unavailableClientError to keep chatWithStats small.

chatrun has 100% test coverage; full suite passes with -race.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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