Review and align context compression across IM and CLI modes

## Goal Review and align context compression behavior between IM mode and CLI mode so both modes preserve equivalent user-visible and runtime semantics. ## Scope - Existing CLI context compression flow - Existing IM/gateway context compression flow - Differences in episode/session boundaries, message roles, tool traces, and recall inputs - Compression trigger policy and output shape - Tests that prevent mode-specific regressions ## Acceptance Criteria - A review note identifies all meaningful differences between CLI and IM context compression. - Intentional differences are documented; accidental differences are removed or tracked. - CLI and IM use shared policy/helpers where practical. - Tests cover representative compression scenarios in both modes. - The result preserves compatibility with prefix-cache reuse work.