refactor(#1281): T2 — migrate 12 single-leaf callers off the core spine (batch 1) (#1282)

Per the T1 design rubber-duck, batch by FILE so each tranche drops
convergence-lint allowlist entries. Migrate 12 files' core imports to the
leaf modules directly (behaviour-identical — leaves are the objects core
re-exports by reference):
- io (output/error/ERROR_REASON): agent-command-router, capability-state,
  capability-writer, frontmatter, gsd2-import, learnings, loop-resolver,
  task-command-router
- roadmap-command-router -> config-loader; workstream-inventory -> core-utils
- milestone, verify -> their full leaf sets (both were multi-leaf, not
  single-leaf as first scoped; migrated completely)

All 12 files now import zero core symbols and are removed from the
allowlist (30 -> 18). core.cts re-exports untouched (still serve the
remaining 18 files); teardown is T-final. Stale core.cjs docstrings in the
migrated files corrected to reference io.cjs. No behaviour change.

Closes #1281

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

scripts/lint-core-spine-imports.allowlist.json

@@ -2,34 +2,22 @@
   "_comment": "Files allowed to import the core re-export spine during the staged retirement (issue #1268). Entries are REMOVED as each tranche migrates a leaf; the file + this lint are deleted in T-final.",
   "allow": [
     "gsd-core/bin/gsd-tools.cjs",
-    "src/agent-command-router.cts",
     "src/audit-command-router.cts",
-    "src/capability-state.cts",
-    "src/capability-writer.cts",
     "src/check-command-router.cts",
     "src/commands.cts",
     "src/config.cts",
     "src/docs.cts",
-    "src/frontmatter.cts",
     "src/gap-checker.cts",
     "src/graphify-command-router.cts",
-    "src/gsd2-import.cts",
     "src/init.cts",
     "src/intel-command-router.cts",
-    "src/learnings.cts",
-    "src/loop-resolver.cts",
-    "src/milestone.cts",
     "src/phase.cts",
     "src/profile-output.cts",
-    "src/roadmap-command-router.cts",
     "src/roadmap.cts",
     "src/state.cts",
-    "src/task-command-router.cts",
     "src/template.cts",
     "src/uat.cts",
     "src/verification.cts",
-    "src/verify.cts",
-    "src/workstream-inventory.cts",
     "src/workstream.cts"
   ]
 }