fix(agent): Unify model request retry policies

[wsp1911]

Share a ten-attempt budget, error classification, and backoff
between ordinary model requests and context compression.

• Remove nested retries from compression requests.
• Retry empty summaries and tool-call responses within the same
  budget, including malformed tool calls accompanied by text.
• Replan immediately on context overflow, granting each new plan
  ten attempts and limiting compression to four plans.
• Remove local summary reconstruction and its configuration.
• Fail the turn when compression fails instead of continuing
  with uncompressed context or an incomplete local summary.
• Preserve existing context and compression counts on failure.
• Cover retry budgets, replanning, cancellation, and state retention.