fix: prevent false link-check failures and harden health/workflow errors

Address three review findings from the Go rewrite:\n\n- checker: update PartitionLinks to only classify HTTP(S) URLs as external links.\n  This skips markdown-relative targets (for example  and anchors) so\n  non-URL entries are no longer sent to HTTP validation and do not produce\n  deterministic "unsupported protocol scheme" failures.\n\n- health command: stop ignoring LoadHealthCache errors.\n  Return a user-facing "load cache" error when the cache file is unreadable\n  or invalid instead of allowing a nil cache panic on Merge.\n\n- broken links workflow: remove masked execution behavior from the link check\n  step. Capture awesome-docker check exit code, set has_errors=true on any\n  non-zero exit, and expose the exit code in the generated issue body so\n  checker failures are visible and cannot incorrectly close the tracking issue.\n\nTest coverage updates:\n- extend checker partition test to include markdown-relative/anchor targets\n  and verify they are not treated as external URLs.\n- add cache test for invalid YAML load failure.

internal/checker/github_test.go

@@ -41,6 +41,8 @@ func TestPartitionLinks(t *testing.T) {
 		"https://example.com/tool",
 		"https://github.com/moby/moby",
 		"https://github.com/user/repo/issues",
+		"dozzle",
+		"#projects",
 	}
 	gh, ext := PartitionLinks(urls)
 	if len(gh) != 2 {