Apply Observational Memory style context management in DSH

Key features

• Automatic compressionmain session only, blocks serial execution at agent/pre-step.
• Borrowing feedback/record eventsThe plug-in's log-only session events (warning and observation compression markers) are written to the text field of type feedback/record known to the host (om:1: prefixed JSON envelope, see src/om-event.ts).
• When the net pressure reaches observeThresholdTokens and then tailMessageCount complete messages are accumulated, all messages before the trigger point are digested into new blocks and the corresponding message range is accurately replaced
• When the total of all block tokens reaches reflectThresholdTokens, all block text is combined into a single block and sent to the summary, and merged into a more compact summary.
• Pending marks are persisted as log-only session events and restored from the log after restart