fix(#1223): install scripts/fix-slash-commands.cjs so gsd-tools loads (#1240)

* fix(#1223): install scripts/fix-slash-commands.cjs so gsd-tools loads

Before this fix, bin/install.js copied scripts/changeset/ and scripts/lib/
into the runtime config dir but omitted scripts/fix-slash-commands.cjs.
gsd-core/bin/lib/command-roster.cjs requires this file at module load via
require('../../../scripts/fix-slash-commands.cjs'), so every gsd-tools
command crashed with MODULE_NOT_FOUND on every installed runtime.

Four changes:
- bin/install.js copy step: copy fix-slash-commands.cjs into <configDir>/scripts/
  with source-missing hard-fail and verifyFileInstalled smoke check
- bin/install.js writeManifest: track scripts/fix-slash-commands.cjs (not
  covered by the changeset/lib subdir loops)
- bin/install.js uninstall: best-effort unlinkSync before scripts/ rmdir
- scripts/fix-slash-commands.cjs readCmdNames(): wrap readdirSync in
  try/catch returning [] so skill-based/global installs without a local
  commands/gsd/ directory do not throw ENOENT

Tests added to tests/install.test.cjs (6 new tests):
- smoke: install() copies fix-slash-commands.cjs
- e2e: spawned gsd-tools.cjs does not crash with MODULE_NOT_FOUND
- manifest: writeManifest() tracks the file
- uninstall: uninstall() removes the file
- readCmdNames unit: export returns an array
- readCmdNames spawn: absent COMMANDS_DIR returns exit 0 (no throw)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#1223): backfill changeset PR number (#1240)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

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

.changeset/1223-installer-fix-slash-commands-missing.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1240
+---
+**gsd-tools no longer crashes at load on a fresh install** — the installer omitted `scripts/fix-slash-commands.cjs`, which `command-roster` requires at module load, so every `gsd-tools` command failed with MODULE_NOT_FOUND. The installer now ships it (with a smoke assertion), and `readCmdNames()` tolerates a missing commands directory. (#1240)