fix(linkindex): stop trimming leading/trailing hyphens from GithubSlug

[willkg]

Summary

• GithubSlug trimmed leading/trailing hyphens with strings.Trim(s, "-"), but the real github-slugger (the package GitHub itself extracted from its rendering pipeline) trims nothing at all -- not a punctuation-derived hyphen, and not even literal leading/trailing whitespace.
• Concretely: GithubSlug("--json output") returned "json-output", but GitHub's real anchor for that heading is --json-output -- which is exactly what README.md's own [--json output](#--json-output) links assume, and markfluence check README.md reported both as "anchor not found" until this fix.
• Removed the trim entirely rather than narrowing it, since a hyphen produced by whitespace-to-hyphen conversion and a hyphen already present in the heading text are the same character by the time trimming would run -- there's no way to "only trim whitespace-derived hyphens" without restructuring the function, and the reference implementation doesn't try to.

Test plan

• make check (vet, fmt-check, test, build, lint) passes
• Updated internal/linkindex/slug_test.go cases to the verified-correct values, including the "---" case the issue flagged as needing re-verification
• Confirmed end to end: markfluence check README.md no longer reports "anchor not found" for the #--json-output links

Fixes #120