Fix automatic compaction dropping the newest user turn

[elrensmin]

Motivation

_first_recent_context_index decides where automatic compaction starts keeping rows. Its "keep whole user turns" logic could move the boundary past the last user message, silently destroying the most recent context:

• [user, assistant, user, assistant] kept only the final assistant reply, dropping the newest user prompt.
• [user, assistant, toolResult, toolResult] compacted the entire context including the newest turn.

Summary of behavior changes

• The compaction boundary is now clamped to the last user message, so the newest user turn and its reply are always retained.
• When nothing can be kept without dropping the newest turn, automatic compaction is skipped instead of compacting everything.

Tests / checks

• Added pure-function regression tests for the boundary cases in tests/test_coding_session.py.
• CI checks clean

Notes

• Localized to _first_recent_context_index in tau_coding.session; no cross-layer changes.
• Added dev-notes/compaction-keeps-newest-user-turn.md.