fix: make FileFilter patterns case-insensitive

[Itachi7011]

Summary

Fixes #844.

FileFilter.IsUserIncluded and FileFilter.IsUserExcluded lowercased
the file path before matching, but did not lowercase the user-provided
pattern.

This caused patterns containing uppercase letters such as
README.md and **/CHANGELOG.md to fail to match.

Changes

• Lowercase expanded exclude patterns before doublestar.Match
• Lowercase expanded include patterns before doublestar.Match
• Extended TestFileFilter_CaseInsensitive with uppercase-pattern
  regression coverage

Testing

go test ./internal/config/rules/...

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[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 1 selected item(s).

[wu21-web]

No issues found.

[lizhengfeng101]

@Itachi7011 Nice first contribution — the fix is exactly right, and I appreciate that you kept it minimal instead of refactoring around it. I pulled the branch locally: tests pass, and I confirmed the new assertions genuinely fail once system_rules.go is reverted, so the regression coverage is real. I also grepped every doublestar.Match call site — these two were the only remaining asymmetric ones, so nothing was missed (allowlist/allowed_ext.go lowercases its patterns at init, and diff/git.go is case-sensitive on purpose since that's gitignore semantics).

Three small suggestions, none of them blocking:

1. Use mixed-case paths in the test. The new assertions pair an uppercase pattern with an all-lowercase path:

if !f.IsUserIncluded("docs/changelog.md") { ... }
if !f.IsUserExcluded("readme.md") { ... }

That does catch the regression, but it sidesteps the spelling people actually hit — pattern and path both containing uppercase, e.g. --exclude packages/backend/convex/seedVolume.ts against a file named seedVolume.ts. The repro in #844 is written that way. The real-world spelling still fails before the fix, and additionally pins the user-facing case:

if !f.IsUserIncluded("docs/CHANGELOG.md") { ... }
if !f.IsUserExcluded("README.md") { ... }

2. Add one brace-expansion case. The only interaction still untested is the ordering of ToLower and expandBraces. You went with expand-then-lowercase — correct, and it mirrors Resolve — but nothing currently locks that in. One extra pattern would:

Exclude: []string{"**/*.{Go,Java}"},  // assert "pkg/Main.JAVA" is excluded

3. State the contract in the doc comments. Neither IsUserExcluded nor IsUserIncluded mentions being case-insensitive, whereas allowlist.IsExcludedPath spells it out ("The check is case-insensitive."). Given this bug was precisely a half-implemented version of that unwritten contract, one line would help keep it from drifting back:

// IsUserExcluded reports whether the given path matches any user exclude pattern.
// The check is case-insensitive: both path and pattern are lowercased.

Thanks again — clean, well-scoped patch.

[Itachi7011]

Thanks for the detailed review! I appreciate the suggestions. I'll keep these in mind. Glad to hear the fix and regression coverage look good. 🙏

[lizhengfeng101]

@Itachi7011 I'd like to see your updates regarding these three small suggestions.

[lizhengfeng101]

LGTM

[Itachi7011]

Thanks for the review! I've addressed all three suggestions:

• Updated the regression cases to use mixed-case paths.
• Added brace-expansion coverage with an uppercase pattern.
• Documented the case-insensitive behavior in both FileFilter methods.

I also ran go test ./... locally, and all tests pass.

[Fanzzzd]

Congrats on landing your first contribution, @Itachi7011 — clean fix, regression test included, merged within a day. Exactly how it should go. Welcome aboard 🎉