Prevent terminal safe-output loss on budget exhaustion - #67470

Scheduled and PR workflows can exhaust their context or invocation budget before recording an outcome, leaving failures classified as unknown. Failure reports also lack the exhausted budget and the last tool call.

• Prompt guardrails: Add an early budget check and a final-turn reminder to stop analysis and emit a terminal safe output.
• Failure diagnostics: Include exhaustion classification, consumed budget, and last tool call in report_incomplete details. For example:

  Failure classification: prompt_exhaustion
  Budget consumed: effective token limit
  Last tool call: github.get_file_contents
  

• Regression coverage: Verify exhaustion still produces report_incomplete and its diagnostics reach the failure issue context.

• Fixes [AW Top 10] 01 Stop agents ending without a terminal safe output #67446