Context

PR #1966 (EPIC-02) extracted Agent fields into sub-structs in agent/state/mod.rs. These sub-structs currently have no direct unit tests — they are only tested indirectly via agent integration tests.

Found by: reviewer (SUG-02)

Acceptance criteria

• Unit tests for InstructionState, ExperimentState, CompressionState, MessageState, SessionState construction and field access
• Tests in agent/state/tests.rs or agent/state/mod.rs inline module