v1.13.9: compaction overflow trigger — 0.85 × ctx_max early trigger

Opencode pattern (session/overflow.ts): fire compaction at 85% of
ctx_max, replacing the v1.11.0-era `ctx_max - 20_000` formula.

Old formula: usable = ctx_max - 20_000
  - ctx=262144 → trigger at 242144 (92.4%) — only 7.6% headroom
  - ctx=100000 → trigger at  80000 (80.0%)
  - ctx= 32000 → trigger at  12000 (37.5%) — over-eager
  - ctx<=20000 → trigger at      0 — never fires

New formula: usable = floor(0.85 * ctx_max)
  - ctx=262144 → trigger at 222822 (85.0%) — 15% headroom for summarizer
  - ctx=100000 → trigger at  85000 (85.0%)
  - ctx= 32000 → trigger at  27200 (85.0%)
  - ctx=  8192 → trigger at   6963 (85.0%)

Ratio gives consistent headroom at any context scale. The qwen3.6
daily driver gets ~19k tokens more breathing room before overflow;
small-ctx models no longer degenerate to never-triggering.

usable() is the only consumer of COMPACTION_BUFFER → constant deleted.
New EARLY_TRIGGER_RATIO constant takes its place.

isOverflow() and the maybeFlagForCompaction() call site at
payload.ts:184 are unchanged — formula swap is internal to compaction.ts.
payload.ts comment touched only to drop the stale COMPACTION_BUFFER
reference (PRUNE_TRIGGER_TOKENS stays at 20k as the prune-freed
threshold; independent of the overflow formula).

Tests: 4 new usable() corner cases (262k/100k/8k/zero+negative), plus
5 isOverflow() numbers shifted to match the 85k budget at ctx=100k.
195/195 server tests pass (was 194).

Smoke: ratio math verified by unit tests at all four corners. Live
cap-hit verification deferred — requires accumulating >222k tokens
in a session under qwen3.6-35b-a3b-mxfp4 (was >242k pre-fix); will
surface organically in extended use.

apps/server/src/services/inference/payload.ts

@@ -199,10 +199,13 @@ export async function maybeFlagForCompaction(
   );
   if (!overflow) return;
 
-  // v1.13.4: try the cheap prune first. If it freed at least the buffer
-  // worth of tokens (PRUNE_TRIGGER_TOKENS, identical to COMPACTION_BUFFER),
-  // we're below the threshold again — skip flagging summarize for the next
-  // turn. The next turn's overflow check will re-evaluate from scratch.
+  // v1.13.4: try the cheap prune first. If it freed at least
+  // PRUNE_TRIGGER_TOKENS (20k) worth of context, we're below the threshold
+  // again — skip flagging summarize for the next turn. The next turn's
+  // overflow check will re-evaluate from scratch.
+  // v1.13.9: the overflow trigger above is now 85% of ctx_max (was
+  // ctx_max - 20k). PRUNE_TRIGGER_TOKENS stays at 20k as the prune-freed
+  // threshold — independent of the overflow formula.
   // Prune failures (DB errors etc.) propagate so the surrounding inference
   // path sees them; the catch in finalizeCompletion / executeToolPhase
   // doesn't shield this — by design, we want to know if prune is broken.