fix(#751): recognise config-set prototype-pollution guard in CodeQL + test dynamic-key vectors (#752)

CodeQL alert #26 (js/prototype-pollution-utility) kept firing on setConfigValue
because its dataflow does not trace the #663 Set-based, pre-loop keys.some(...)
forbidden-key check as a sanitising barrier on the write site.

- src/config.cts: replace the Set + pre-loop check with inline literal
  comparisons (key === '__proto__' || 'prototype' || 'constructor') on the exact
  key used to index `current`, immediately before each write (intermediate keys
  in the descent loop, plus the final key). Same forbidden set, same error
  message and ERROR_REASON.CONFIG_PARSE_FAILED — behaviour unchanged from #663,
  but the barrier is now CodeQL-recognised.
- tests/config.test.cjs: add regression tests for schema-valid dynamic-prefix
  keys (agent_skills.__proto__, agent_skills.constructor, agent_skills.prototype,
  features.__proto__, review.models.constructor) that pass the isValidConfigKey
  schema gate and reach the guard. Each asserts the guard's own message fires
  (not the schema gate's "Unknown config key") and Object.prototype is not
  polluted. The prior #663 tests never reached the guard — their keys are
  rejected by the schema gate first — so the guard's real attack surface was
  untested.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/751-config-prototype-pollution-codeql-alert-26.md

@@ -0,0 +1,5 @@
+---
+type: Security
+pr: 752
+---
+**`gsd-tools config-set` prototype-pollution guard hardened and regression-tested.** The guard that blocks `__proto__`, `prototype`, and `constructor` segments in dotted config keys now uses inline literal comparisons at each property-write site (instead of a pre-loop `Set` check), so CodeQL's `js/prototype-pollution-utility` analysis recognises it as a sanitising barrier and code-scanning alert #26 clears. Runtime behaviour is unchanged from #663. Added regression tests that drive schema-valid dynamic-prefix keys (`agent_skills.__proto__`, `agent_skills.constructor`, `features.__proto__`, `review.models.constructor`) all the way to the guard — these reach `setConfigValue` past the schema gate and were previously the guard's only untested attack surface. (#751)