fix(#4120): replace shellcheck npm dep with dependency-free downloader (#4121)

* fix(#4120): replace shellcheck npm dep with dependency-free downloader

The `shellcheck` devDependency (added in #4109) pulled in decompress@4.2.1
for archive extraction, which carries an unpatched CRITICAL zip-slip
vulnerability (GHSA-mp2f-45pm-3cg9, CVSS 9.1) plus two moderate findings.
decompress's latest published version IS the vulnerable one -- no patched
release exists upstream, so npm audit fix cannot resolve this by upgrading.

Removes the shellcheck package entirely and replaces its role with
scripts/lib/shellcheck-fetch.cjs: a small downloader using only Node's
built-in https/zlib plus a hand-written tar-entry reader, fetching a pinned
koalaman/shellcheck release directly from GitHub releases. The reader never
uses an archive-supplied name as a filesystem path (the exact defect class
decompress had) -- it only returns the matched entry's bytes; the caller
writes those bytes to a path it constructs itself. Bounds the download with
a 30s-per-hop timeout, consistent with the ShellCheck subprocess's own
timeout. Covers linux/darwin on x86_64/aarch64, matching this repo's actual
CI (lint-tests runs only on ubuntu-latest) and local dev needs; Windows
fails with a clear, honest error rather than silently misbehaving.

Adds tests/lint-workflow-shellcheck-fetch.test.cjs covering the tar-parser
(unit cases plus a fast-check property test per CLAUDE.md's parser-testing
requirement), a security behavioral pin confirming traversal-style entry
names are treated as opaque strings never filesystem paths, and boundary
coverage for the redirect-following logic's MAX_REDIRECTS limit
(limit-1/limit/limit+1, via an injectable transport, no real network I/O).

npm audit: 0 vulnerabilities (was 1 critical + 5 moderate). The lint script
reproduces the identical result against the current tree:
"212 pre-existing finding(s) from baseline, 0 new" -- no behavior
regression, no baseline changes needed.

* fix(#4120): register shellcheck-fetch.cjs with the installer

scripts/lib/shellcheck-fetch.cjs shipped without being added to
GSD_SCRIPTS_LIB_FILES in bin/install.js, which would have left it orphaned
on uninstall and broken the golden install-tree fixtures for every runtime.
Adds the entry and regenerates the 19 affected fixtures via
npm run gen:install-tree.

* docs(#4120): add changeset for the decompress CVE fix

---------

Co-authored-by: sim <sim@local>

tests/fixtures/install-tree/opencode.json

@@ -512,6 +512,7 @@
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
   "scripts/lib/ndjson-reporter.cjs",
+  "scripts/lib/shellcheck-fetch.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",