feat(server): SPOT-derived prompt budget — MAX_PROMPT_CHARS default follows models.json (ADR 0009)

Maintainer directive (2026-07-18): the hand-set 150,000-char default (~37.5k English
tokens) is obsolete in the long-context era. Instead of a new constant that would rot
the same way, the default now derives from the SPOT:

  MAX_PROMPT_CHARS (default) = max(models.json contextWindow) x 3 chars/token
                             = 200000 x 3 = 600,000 chars today (~150-200k tokens)

x3 is the CJK-safe multiplier: English runs ~4 chars/token, CJK ~1-1.5, so a
1M-token-derived char cap would let CJK text sail past the model's real window into
an upstream rejection; at x3 the cap fires at roughly the model's true window and OCP
truncates gracefully (tail-first) instead. Pure derivePromptCharBudget() in
lib/prompt.mjs with a 150k floor guarding degenerate SPOT states (empty models[],
absent contextWindow) - a zero budget would truncate every request to nothing.

CLAUDE_MAX_PROMPT_CHARS (env) and the runtime settings API remain ABSOLUTE overrides;
derivation applies only when neither is set. If models.json ever advertises a larger
window (e.g. 1M for the 1M-native models), the budget scales automatically - that
advertisement is a separate deliberate decision (quota burn, OpenClaw compaction, TUI
paste limits) explicitly NOT made here; see ADR 0009.

Behavior change (intended): requests between 150k and 600k chars previously truncated
now pass through whole - longer TTFT + higher quota use for those requests. Truncation
mechanism/logging unchanged; only the default's provenance changed.

ALIGNMENT.md Rule 2: no cli.js citation applies - the truncation guard is OCP-internal
prompt shaping; no endpoint, header, or wire field changes.

Tests: +4, doubly mutation-proven (max->min fails the largest-window test; dropping
the floor fails the floor test). Suite 347 passed / 0 failed. ADR 0009 + index row +
README env-table row included (release_kit: env var default change documented).

Co-Authored-By: Claude <claude-opus-4-8> <noreply@anthropic.com>

lib/prompt.mjs

@@ -15,3 +15,29 @@ export function appendOperatorPrompt(base, operatorAppend) {
   const op = typeof operatorAppend === "string" ? operatorAppend.trim() : "";
   return op ? `${base}\n\n${op}` : base;
 }
+
+// Derive the default prompt-char budget from the models.json SPOT (ADR 0009).
+//
+// The old default was a hand-set constant (150000 chars ≈ 37.5k English tokens) from the
+// 200k-window era — silently far below what the advertised contextWindow promises. Instead
+// of picking a new constant that will also rot, the default now FOLLOWS the SPOT:
+//
+//   budget = max(models[].contextWindow) × charsPerToken
+//
+// charsPerToken = 3 is deliberately conservative: English runs ~4 chars/token, CJK ~1–1.5.
+// At ×3, a 200k-token window yields 600,000 chars — full window for English, and CJK text
+// hits the model's real window at roughly the same point the cap fires, so we truncate
+// (graceful, tail-first) rather than let the upstream reject the request outright.
+//
+// The floor guards the degenerate cases (empty/missing models[], absent contextWindow):
+// fall back to the historical constant rather than 0 — a zero budget would truncate every
+// request to nothing, which is fail-OPEN in the "serve garbage" sense. CLAUDE_MAX_PROMPT_CHARS
+// remains an absolute operator override at the call site (server.mjs); this function is only
+// the unset-env default.
+export function derivePromptCharBudget(models, { charsPerToken = 3, floor = 150000 } = {}) {
+  const windows = (Array.isArray(models) ? models : [])
+    .map(m => m?.contextWindow)
+    .filter(w => Number.isFinite(w) && w > 0);
+  if (windows.length === 0) return floor;
+  return Math.max(floor, Math.max(...windows) * charsPerToken);
+}