refactor(#859): extract CLI I/O primitives from core.cts into io.cts (#864)

ADR-857 rollout phase 1. Move the CLI I/O primitives — output(), error(),
ERROR_REASON, setJsonErrorMode/getJsonErrorMode, and the output() large-payload
temp-file spillover helpers (GSD_TEMP_DIR, ensureGsdTempDir, reapStaleTempFiles)
— out of the 2271-line core.cts god-module into a new, small src/io.cts. core.cts
re-exports them so existing consumers are unaffected (behavior-preserving).

Repoint src/profile-pipeline.cts to import output/error/reapStaleTempFiles from
io directly; graphify/intel/audit were verified not to import these symbols. Net:
the leaf feature modules no longer depend on core just for I/O — the enabling
first cut toward Capability extraction.

New-CLI-module checklist: .gitignore (bin/lib/io.cjs), eslint.config.mjs ignores,
INVENTORY.md count 90→91 + io.cjs row, INVENTORY-MANIFEST.json, ARCHITECTURE.md
core.cjs/io.cjs rows, CONTEXT.md "I/O Module" glossary entry. Adds tests/io.test.cjs
(28 behavioral tests incl. shim-identity and the @file: spillover branch).

Gates: lint, code-review, security-review, codex adversarial-review, and
gsd-test-both (14742 pass on Mac + Linux Docker, 0 fail) all green.

Closes #859

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

docs/INVENTORY-MANIFEST.json

@@ -1,5 +1,5 @@
 {
-  "generated": "2026-06-07",
+  "generated": "2026-06-08",
   "families": {
     "agents": [
       "gsd-advisor-researcher",
@@ -301,6 +301,7 @@
       "installer-migration-report.cjs",
       "installer-migrations.cjs",
       "intel.cjs",
+      "io.cjs",
       "learnings.cjs",
       "legacy-cleanup.cjs",
       "milestone.cjs",