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>

src/task-command-router.cts

@@ -9,8 +9,8 @@
 import fs from 'node:fs';
 import path from 'node:path';
 // eslint-disable-next-line @typescript-eslint/no-require-imports
-import core = require('./core.cjs');
-const { output, error, ERROR_REASON } = core;
+import ioMod = require('./io.cjs');
+const { output, error, ERROR_REASON } = ioMod;
 
 // ─── Types ────────────────────────────────────────────────────────────────────