P4: GitHub issue/PR autolinks (#123) non-interception

[RE-codes]

Story

As a vault user, I expect GitHub-specific autolinks like #123 or owner/repo#123 to behave as ordinary text (or as a tag if Obsidian treats #123 as a tag).

Tasks

• Verify that the plugin does not try to resolve #123 as a GFM heading fragment (it isn't a markdown link).
• Add a fixture and confirm in all modes.
• If #123 ever reaches the plugin's resolver, fix the bail-out logic.

Acceptance Criteria

• #123 and owner/repo#123 are not intercepted.
• Matrix P4 row updated.

References

• Parent epic: Epic: P4 Lower-Priority Readiness #93