chore(release): v1.20.0.0 — cross-platform hardening, curated Windows lane

Cross-platform hardening. Mac + Linux full, curated Windows lane added.

Workspace-aware queue at ship time:
- v1.17.0.0 claimed by garrytan/setup-gbrain-run (PR #1234)
- v1.19.0.0 claimed by garrytan/browserharness (PR #1233)
- This branch claims v1.20.0.0 (next available slot)

(Initially bumped to v1.18.0.0 during plan-mode implementation; rebumped to
v1.20.0.0 at /ship time when gstack-next-version detected the queue had moved.)

Headline numbers (full release-note in CHANGELOG.md):
- 2 new shared resolvers: bin/gstack-paths (61 LOC), browse/src/claude-bin.ts (73 LOC)
- 8 skills migrated off inline state-root chains
- 5 hardcoded claude spawn sites rewired through the shared resolver
- 75 LOC of fork-side reimplementation replaced by Bun.which()
- 103 of 128 free tests run on windows-latest (curated, ~80%)
- +31 new unit tests + 3 new invariants
- AGENTS.md inventory grows from 21 to 40+ skills

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "1.18.0.0",
+  "version": "1.20.0.0",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",