fix(agent): preserve newest turn in chronological conversation lists - #224

Summary

Long chronological conversation state is serialized newest-last. When it exceeds the available state budget, build_sequence defaults to right truncation (st[:room]), so the newest turn can be silently dropped while Agent.system_one still returns a normal decision.

This patch preserves the most recent conversational intent for the public list-shaped state contract.

Change

• Detect list-shaped state in Agent.system_one
• Pass truncate_left=True for conversation turn lists
• Keep existing truncation behavior unchanged for string and dict state
• Leave the build_sequence API and default unchanged

Why this shape

Agent.system_one documents list state as a conversation turn list, so the newest turn is at the tail. Preserving the tail is the smallest change that protects recent conversational intent without changing non-conversation callers.

Validation

• Added 5 regression tests covering list vs. string truncation behavior
• Full model-free test run: 556 passed, 0 failed
• Reproduced the failure before the fix on the pinned upstream base

This is orthogonal to #181, which covers truncation metadata reporting. The older #2 explored broader truncate_left plumbing; this PR keeps the change focused on the public conversation-list behavior.