test(coverage): add unit tests across all packages; remove dead code

- Added comprehensive table-driven test suites for all packages:
  cmd/glint, cicontext, fetcher, graph, linter, model, resolver.
  Coverage reaches 98%+ statement coverage across the codebase.
- Replaced os.Exit calls in cmd/glint with an `exit` variable so tests
  can capture exit codes without terminating the test process.
- Removed unreachable code found during coverage analysis:
  dead guard in cicontext.parseRegexLiteral; dead len(jobs)==0 branch
  in graph.Pipeline; skipWin struct field and dead continue in
  graph.convertToPNG; pipelineSVG return type simplified to string.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/glint/explain.go

@@ -18,7 +18,8 @@ func cmdExplain(args []string) {
 	entry, ok := linter.RuleCatalog[ruleID]
 	if !ok {
 		fmt.Fprintf(os.Stderr, "glint explain: unknown rule %q\n\nRun 'glint explain' to list all rules.\n", ruleID)
-		os.Exit(2)
+		exit(2)
+		return
 	}
 	printRuleEntry(ruleID, entry)
 }