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/capability-writer.cts

@@ -13,7 +13,7 @@
  *   - re-resolve: always calls resolveCapabilityRuntimeState for the return value
  *
  * Dependencies (leaf modules only — no circular risk):
- *   - ./core.cjs               (output, error)
+ *   - ./io.cjs                 (output, error)
  *   - ./capability-state.cjs   (resolveCapabilityRuntimeState, _resolveManifest, _resolveCommandsGsdDir)
  *   - ./surface.cjs            (readSurface, writeSurface, applySurface)
  *   - ./install-profiles.cjs   (readActiveProfile)
@@ -23,8 +23,8 @@
  */
 
 // eslint-disable-next-line @typescript-eslint/no-require-imports
-import core = require('./core.cjs');
-const { output: coreOutput, error: coreError } = core;
+import ioMod = require('./io.cjs');
+const { output: coreOutput, error: coreError } = ioMod;
 
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import capabilityStateMod = require('./capability-state.cjs');