docs(hooks): correct PreToolUse → PostToolUse in readSessionCost docblock

greptile P2 nitpick: the previous commit's docblock said "on every
PreToolUse hook" but the module header (and the actual hook wiring
in `hooks/hooks.json`) identifies this script as a PostToolUse
hook — it runs *after* each tool invocation to update the running
session aggregate. One-word typo, no behavior change.

scripts/hooks/ecc-metrics-bridge.js

@@ -89,7 +89,7 @@ function extractFilePaths(toolName, toolInput) {
  *
  * costs.jsonl is append-only and unbounded today (no rotation in
  * cost-tracker.js). At a typical ~150 bytes per row, even 100k rows
- * is ~15 MB and a single sync read on every PreToolUse hook is in
+ * is ~15 MB and a single sync read on every PostToolUse hook is in
  * the low milliseconds. If rotation lands later, this scan becomes
  * even cheaper.
  */