fix(bin): delete zero-caller scripts; make host-config-export's docstring honest

- bin/gstack-open-url (14 lines): announced in a CHANGELOG entry, wired into
  nothing, ever. bin/gstack-platform-detect (27 lines): zero callers, and its
  hand-rolled host list was already stale (SLATE_HOST.md cites it as a
  problem). Note: the deprecated gstack-brain-consumer/reader pair the audit
  flagged was already deleted upstream in v1.63 with a stay-deleted tripwire.
- scripts/task-emission-schema.ts (61 lines): a typed schema module nothing
  imported; the tasks-section comment now documents the JSONL fields inline.
- scripts/host-config-export.ts claimed to be the 'shell bridge for the bash
  setup script' — setup never calls it (its hand-rolled host lists drifting
  is a known follow-up). Docstring now states what it IS: a standalone,
  test-pinned query CLI not yet wired into setup. Its validateValue +
  CLI_REGEX/PATH_REGEX internals were dead (defined for a guarantee the
  header claimed but nothing enforced).
- KEPT deliberately: scripts/preflight-agent-sdk.ts — a documented manual
  diagnostic (CONTRIBUTING.md + USING_GBRAIN_WITH_GSTACK.md reference it).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/resolvers/tasks-section.ts

@@ -4,7 +4,9 @@
  *   {{TASKS_SECTION_EMIT:<phase>}}     — per-skill task emission + JSONL write
  *   {{TASKS_SECTION_AGGREGATE}}        — autoplan aggregation across all phases
  *
- * Schema for the JSONL artifact lives in scripts/task-emission-schema.ts.
+ * JSONL artifact fields: phase, run_id, branch, commit, id, priority,
+ * component, files, effort_human, effort_cc, title, source_finding
+ * (consumed by /autoplan's aggregator).
  */
 
 import type { TemplateContext, ResolverFn } from './types';