feat(config): register spend-control keys + validate spend.posture (#2139)

Adds spend.posture + the five previously --force-only spend knobs to
KNOWN_CONFIG_KEYS so `config set` accepts them directly (removes the
archaeology the issue complained about), and rejects invalid spend.posture
values at set time with a paste-ready hint.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/commands/config.ts

@@ -181,6 +181,20 @@ export async function runConfig(engine: BrainEngine, args: string[]) {
     const coverageOverride =
       args.includes('--coverage-override') || args.includes('--yes');
 
+    // v0.42.42.0 (#2139): validate spend.posture at set time so a typo
+    // ('tokenMax', 'max') doesn't silently fall back to gated.
+    if (key === 'spend.posture') {
+      const { isValidSpendPosture } = await import('../core/spend-posture.ts');
+      if (!isValidSpendPosture(value)) {
+        console.error(
+          `[config] spend.posture must be 'gated' or 'tokenmax' (got '${value}').\n` +
+          `[config]   gbrain config set spend.posture tokenmax   # cost gates become informational\n` +
+          `[config]   gbrain config set spend.posture gated       # default — gates enforce`,
+        );
+        process.exit(1);
+      }
+    }
+
     if (key === 'embedding_columns') {
       try {
         const parsed = JSON.parse(value);

src/core/config.ts

@@ -892,6 +892,15 @@ export const KNOWN_CONFIG_KEYS: readonly string[] = [
   // Link resolution (issue #972)
   'link_resolution',
   'link_resolution.global_basename',
+  // Spend controls (v0.42.42.0, issue #2139). Previously `--force`-only — the
+  // operator had to discover these by reading source. Registered so `config
+  // set` accepts them directly. See docs/operations/spend-controls.md.
+  'spend.posture',
+  'sync.cost_gate_min_usd',
+  'sync.federated_v2',
+  'embed.backfill_cooldown_min',
+  'embed.backfill_max_usd_per_source_24h',
+  'embed.backfill_max_usd',
 ];
 
 /**