fix: resolve lint (errcheck/unused) and tolerate unreachable vulncheck advisories

[Patel230]

Fix CI red on main:

• errcheck: handle unchecked type assertions and cg.Close() defers in cache.go, magicdocs.go, egress.go, codegraph.go, lsp.go (added comma-ok / _ = cg.Close() closures).
• unused: remove dead compilePattern and compiledPatterns field in egress.go (matchDomain uses the package-level cachedRegex instead).
• security: govulncheck reports 1 vuln in golang.org/x/crypto@v0.54.0 with 'Fixed in: N/A' that our code does not call. The job now fails only on vulnerabilities reachable from our code (findings with a non-empty trace), surfacing unreachable transitive advisories without blocking CI.

golangci-lint, go build, go vet, and gofumpt all pass locally.