fix: bound prompt truncation loop

[Upper9527]

Summary

• bound the oldest-message truncation loop by the number of non-system messages
• preserve system messages and the newest non-system message when the prompt cannot fit
• log the remaining over-budget condition instead of spinning forever
• avoid recalculating the original/result token estimates for logging

Closes #77.

Verification

• pytest -q tests/test_prompt_truncation.py — 2 passed
• uvx ruff@0.12.2 check .
• uvx ruff@0.12.2 format --check .
• full suite: 1 pre-existing dashboard failure, 132 passed, 1 skipped