chore: changeset for context compaction

Author: kevin-dp

.changeset/context-compaction.md

@@ -0,0 +1,26 @@
+---
+'@electric-ax/agents-runtime': minor
+'@electric-ax/agents-server-ui': patch
+---
+
+Context compaction for the agents runtime. Modelled on OpenAI Codex's
+summarization but adapted to the event-sourced timeline (a `context_inserted`
+checkpoint placed at a stored watermark, so reconstruction folds older messages
+into a summary):
+
+- A context-window usage gauge (cache-inclusive `context_input_tokens` +
+  `context_window` persisted per step) and `<token_budget>` notices injected at
+  25 / 50 / 75% usage.
+- Oversized tool-output truncation, and a synchronous mid-turn compaction floor
+  at the 90% hard ceiling (runs before every model step via the adapter's
+  `transformContext` hook).
+- Non-blocking background (turn-end) compaction that starts at 85%: a detached
+  summarize whose checkpoint is applied at the next turn's start, or immediately
+  if it finishes while idle. Each generation uses a watermark-unique checkpoint
+  id so a new run can't supersede a prior completed one. Summarize calls are
+  bounded by a hard timeout.
+- UI: a "Compacting…" indicator (blocking vs. background) and a collapsible
+  "Context compacted" entry in the conversation timeline.
+
+Thresholds are env-tunable (`ELECTRIC_AGENTS_COMPACT_CEILING`,
+`ELECTRIC_AGENTS_COMPACT_BG_CEILING`, `ELECTRIC_AGENTS_COMPACT_MIN_TOKENS`).