P4: GFM extended autolinks and bare URL behavior

[RE-codes]

Story

As a vault user, I want bare URLs and GFM extended autolinks to remain native external behavior — the plugin must not intercept them — and to confirm this is already handled.

Tasks

• Add a fixture with bare URLs (http://x, www.x.com) and a GFM autolink (<https://x>).
• Verify each opens externally without plugin interception in all modes.
• If interception happens (it shouldn't, since these aren't [](url) markdown links), add a non-interception unit test.

Acceptance Criteria

• Bare URLs and autolinks open externally with no plugin interception.
• Matrix P4 autolinks row updated to GREEN.

References

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