feat(reasoning): add CODEGRAPH_OFFLOAD_DISABLE kill-switch and per-call usage log

`CODEGRAPH_OFFLOAD_DISABLE=1` immediately disables the offload for the current
process without touching the persisted config or stored login — useful for A/B
arms or sessions where raw source is preferred.

`CODEGRAPH_OFFLOAD_USAGE_LOG=` appends one JSONL entry per call with token
counts, charged credits, and derived cost (`creditsCharged / 100_000`) so a
harness can attribute CodeGraph AI spend to a single run independently of the
server's cumulative totals. Both features are best-effort and never disrupt the
degradable offload path.

Also fixes the `login` credit display to check `unlimited` before the numeric
balance, so comped/internal accounts don't incorrectly show "0 remaining".

src/bin/codegraph.ts

@@ -1382,8 +1382,14 @@ program
       success('Signed in to CodeGraph AI — managed reasoning is on.');
       try {
         const usage = await fetchUsage();
-        if (usage && typeof usage.remaining === 'number') {
-          info(`  credits: ${usage.remaining.toLocaleString()} remaining`);
+        if (usage) {
+          // Mirror `codegraph usage`'s precedence: a comped/internal account is
+          // flagged `unlimited` (often with remaining:0 when no allowance is set),
+          // so check that before the numeric balance or it reads "0 remaining".
+          if (usage.banned) warn('  Account suspended — contact support.');
+          else if (usage.unlimited) info('  credits: unlimited');
+          else if (typeof usage.remaining === 'number')
+            info(`  credits: ${usage.remaining.toLocaleString()} remaining`);
         }
       } catch {
         /* balance is best-effort */