router: decouple per-run key budget from wallet, add overdraft buffer (#616)

Replaces today's `keyLimitUsd = min(walletBalance, $25)` with population-aware
buffers so users can use 100% of their credits before being paywalled, and
opaque mid-run "more credits" failures (e.g. https://github.com/pullfrog/app/actions/runs/25531633203)
get a clear PR comment instead of a generic stack-trace dump.

Policy matrix:
- Auto-reload accounts: `wallet + autoReloadAmountCents` (default $50, no cap)
- Card + no-autoreload: `wallet + $5` overdraft buffer
- No card: `wallet` (no buffer; existing zero-balance 402 stays)
- OSS: `$10` (unchanged)

Removes the $25 per-run cap entirely. Long Build runs at high-balance
accounts no longer silently cap at $25.

Other changes:
- Classify mid-run OpenRouter "requires more credits, or fewer max_tokens"
  errors as `router_keylimit_exhausted` BillingError so users get an
  actionable PR comment.
- Override OpenCode `max_tokens: 32000` default to `5000` via
  OpenCodeConfig.limit.output. Drops Opus per-call upfront budget reservation
  from ~$2.40 to ~$0.38 — what makes low-wallet runs viable at all.
- Switch `findInitialComment` and `findExistingPaywallComment` to GraphQL
  `issueOrPullRequest(number:) { comments(last: 100) }` (single round trip,
  actually returns newest-100; REST listComments doesn't support sort/direction).
  Also fixes a latent `comments.find()` returning the OLDEST match instead
  of the most recent — now selects max(databaseId).
- Wrap `syncAccountUsage` in `prisma.$transaction` with `SELECT ... FOR UPDATE`
  on the account row. Pre/post-balance reads inside the transaction enable
  deterministic low-balance edge detection (currently logs; will push the
  outreach.low_balance task once #592 lands).

Plan: .cursor/plans/router-low-balance-paywall.plan.md (in companion wiki-billing branch)

utils/providerErrors.ts

@@ -36,3 +36,24 @@ export function detectProviderError(text: string): string | null {
   }
   return null;
 }
+
+/**
+ * OpenRouter's response when the per-run key's remaining budget can't cover
+ * the agent's `max_tokens` reservation. Distinct from a generic provider error
+ * because it's a Pullfrog billing concern, not an upstream outage — the user's
+ * Router wallet ran out (or the key budget was undersized at mint time and the
+ * agent ran out of headroom partway through).
+ *
+ * Match must be specific to this exact OpenRouter error class. Generic "credits"
+ * or "limit" text shows up in unrelated errors and would mis-classify them.
+ *
+ * Sample:
+ *   `APIError: This request requires more credits, or fewer max_tokens.
+ *    You requested up to 32000 tokens, but can only afford 22800.`
+ */
+const ROUTER_KEYLIMIT_EXHAUSTED_PATTERN =
+  /requires more credits.*?fewer max_tokens|requested up to \d+ tokens.*?can only afford/i;
+
+export function isRouterKeylimitExhaustedError(text: string): boolean {
+  return ROUTER_KEYLIMIT_EXHAUSTED_PATTERN.test(text);
+}