refactor(lint): use git-changed for --changed detection

[pyramation]

Summary

src/changed.ts shipped its own copy of merge-base changed-file detection — default-branch discovery, porcelain parsing, rename arrows, existence filtering. That algorithm now lives in git-changed (0.3.0, zero runtime deps) and is shared with pgpm's bundle-drift check, so this file is the .sql filter over it and nothing else: ~120 lines → ~55, no child_process/fs/path imports left.

-const files = new Set(workingTreePaths(cwd));           // status --porcelain -uall, hand-parsed
-const mergeBase = tryGit(['merge-base', 'HEAD', base], cwd);
-for (const p of tryGit(['diff', '--name-only', '--diff-filter=ACMR', mergeBase, 'HEAD'], cwd)) …
-return { files: abs.filter(existsSync).sort(), base, mergeBase };
+const { paths, base, mergeBase } = gitChangedFiles({ cwd, base, ext });
+return { files: paths, base, mergeBase };

Public surface (resolveChangedBase, changedFiles, changedSqlFiles, ChangedFilesResult) and the CLI are unchanged, --changed semantics are unchanged, and the .sql filter is now ext: '.sql' inside the shared implementation rather than a .endsWith() afterwards.

One deliberate behaviour change. When $GITHUB_BASE_REF names a branch that resolves to no ref at all — neither origin/develop nor develop exists, i.e. a single-branch or shallow fetch — this used to return the bare develop anyway. Every later git call then rejected it and the gate silently degraded to working-tree changes, which in CI (where the branch's work is already committed) means it lints nothing and passes. git-changed only returns refs that resolve, so the base falls through to the repository's default branch and the gate still checks something. The test for that case is updated with the reasoning; the other 73 are untouched and pass.

74 tests green against the published git-changed@0.3.0, and --changed verified end-to-end on a scratch repo (an untracked .sql file is still found — that's the -uall behaviour this originally fixed).

Needs a publish for downstreams to get it, but nothing here changes what --changed does, so it can ride the next release rather than being pulled forward.

Link to Devin session: https://app.devin.ai/sessions/e84444b40007481e9de0285d8b5340b6
Requested by: @pyramation

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[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	git-changed@​0.3.0

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