feat(runtime): replace per-round history trimming with threshold-triggered LLM compaction

Align native context management with the Claude Code / Codex model:
entry-capped tool results, history frozen below the threshold, one
high-quality summary at the wall — instead of the old pipeline that
microcompacted old messages from 60% usage and hid everything past 40
messages behind a snip marker with no summary.

- context pipeline: history below the hard threshold is never rewritten
  (model quality and prompt-cache prefixes depend on byte-identical old
  messages); the 60% tool-aware microcompact and the 40-message history
  snip move to an emergency-only fallback used under overflow pressure
  when the summarizer is unavailable or circuit-broken.
- durable compaction (was a stub): at usage >= context_guard.hard_threshold
  (now 0.90, soft_threshold removed) the old span is folded into a
  9-section summary via the new HistoryCompactor.summarize_runtime_history,
  keeping the system head, the seed user request verbatim on every round
  (injected session-memory/artifact messages shift the stale
  base_prefix_len, so the fold start is structure-aware), and a
  pairing-safe recent tail. A previous summary stays foldable, so exactly
  one summary exists at a time and rounds chain.
- token accounting anchors on the provider-reported prompt size of the
  latest request (max with the local estimate).
- reactive_compaction.circuit_breaker_failures (previously unread) now
  stops repeated summarizer failures; provider overflow errors retry
  through the same pipeline, summary-first.
- tool-result budget clip keeps head and tail instead of tail-chopping.
- chat-side transcripts get the same treatment: new
  MemoryManager.maybe_compact_session_history wires the threshold-gated
  maybe_compact_session into secretary, office_ui dispatcher, and
  context_loader before prompt building, closing the unbounded-growth
  path; dead no-op compactor entries (maybe_compact_after_message,
  should_compact_prompt) removed.

Verified by 13 new tests (history sanctity below threshold, multi-round
single-summary/seed-verbatim/chain invariants, breaker, emergency
fallback, provider-overflow end-to-end recovery) plus a live-provider
probe: multi-round compaction with the model completing correctly from
summarized context. Full suite: 1859 passed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

opc/core/config.py

@@ -585,8 +585,9 @@ class StreamRenderingConfig(BaseModel):
 
 class ContextGuardConfig(BaseModel):
     enabled: bool = True
-    soft_threshold: float = 0.60
-    hard_threshold: float = 0.80
+    # Below this usage ratio the history is never rewritten; at or above it
+    # the runtime folds old messages into one LLM summary (durable compaction).
+    hard_threshold: float = 0.90
     warn_remaining_pct: int = 15
     tool_output_char_budget: int = 12_000
     shell_stdout_char_budget: int = 12_000