fix(lsp): never scan the filesystem root for an opened document's siblings

[devin-ai-integration]

What and why

Since sibling-import indexing landed (#467), Server.indexOpenedDirectory scans the directory of any document opened outside every workspace folder. The cmd/sysml-lsp tests open documents at file:///m.sysml, so the server walked / with its 2000-directory budget. On CircleCI that walk reaches /home/circleci/project/examples and parses thousands of corpus models before the budget runs out: TestStrictFlagServesStrictDiagnostics took over 17 minutes per subtest, and every develop build since #467 has failed Go race tests (no output for 30 min) and Go coverage profile (cmd/sysml-lsp hit the 30 min go test timeout).

indexOpenedDirectory now returns before taking the lock when filepath.Dir(name) is the volume root (/, C:\). A document at the root has no workspace and no plausible sibling set, and the walk from there is unbounded in work even when bounded in directories.

How it was verified

• New TestOpeningFileAtFilesystemRootIndexesNothing in internal/frontend/lsp: opening /m.sysml records no openDirs and publishes no diagnostics.
• go test -count=1 -race -run TestStrictFlagServesStrictDiagnostics ./cmd/sysml-lsp now completes in under 2 s.
• gofmt -l, go vet, go test ./internal/frontend/lsp, python3 scripts/changelog.py check clean.

Checklist

• make test and make lint pass locally (targeted packages; the full suite runs in CI)
• Tests added or updated for the change
• Documentation extended where it already covers the surface (see CONTRIBUTING.md)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (compliance rows need nothing: the census is counted at docs build)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.