feat: log and optionally display compaction events

[just-cameron]

Summary

• Handle compaction_start and compaction_summary SDK message types in both processMessage() and sendToAgent() stream loops
• Log before/after token counts, message counts, and trigger type on every compaction
• Add display.showCompaction config option to optionally show a brief note in the active channel (e.g. "(Context compacted: 197K -> 64K tokens)")
• Add showCompaction to both DisplayConfig and BotConfig.display

Context

Ed T noticed that compaction events never appear in lettabot logs, making it impossible to know if/when automatic compactions fire on the server. Manual trigger worked (197K -> 64K), but there was no way to verify automatic compaction behavior.

The Letta API already sends event_message and summary_message during compaction, but the SDK was silently dropping them. letta-code-sdk#76 fixes the SDK side; this PR consumes the newly-surfaced messages.

Config example

features:
  display:
    showCompaction: true  # optional, default false

Dependencies

• Requires letta-code-sdk#76 to be merged and published

Test plan

• Type-checks clean (tsc --noEmit)
• Verified foreground run filter passes compaction messages through (no runId -> eventRunIds is empty -> not filtered)
• Manual test with a compaction event after SDK bump

Written by Cameron ◯ Letta Code

"Not everything that counts can be counted, and not everything that can be counted counts." -- William Bruce Cameron

[just-cameron]

Blocked on upstream

This PR depends on letta-code-sdk#76, which is itself blocked on a server-side gap: the conversations API endpoint does not emit compaction events. The legacy agents/{id}/messages/stream endpoint works correctly.

Tracking: LET-7913

Once LET-7913 is resolved and SDK letta-ai/letta-agent-sdk#76 is published, this PR needs a version bump commit before merge.