feat(security): security hardening, proxy support, and GL045 HTTP include warning

Security fixes:
- Path traversal guard in include: local: — paths with ../ that escape
  the repo root are rejected instead of reading arbitrary host files
- HTTP timeout (30 s) on all fetcher requests to prevent indefinite hangs
- Response size cap (10 MiB) via io.LimitReader to prevent memory exhaustion
- Cache directory and file permissions tightened to 0700/0600
- LSP Content-Length cap (64 MiB) to guard against DoS from a malicious client

New feature:
- --proxy flag on check, graph, and lsp subcommands; also proxy: key in
  .glint.yml; overrides system HTTP_PROXY / HTTPS_PROXY env vars when set;
  cmdGraph and cmdLSP now also load .glint.yml for proxy/token/url fallbacks

New lint rule:
- GL045 (Warning): include: remote: using plain http:// instead of https://

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

internal/linter/linter.go

@@ -67,6 +67,7 @@ func Lint(p *model.Pipeline, skipped map[string]bool) []Finding {
 	findings = append(findings, checkVariableRefs(p)...)
 	findings = append(findings, checkRulesIfReachability(p)...)
 	findings = append(findings, checkInheritCompleteness(p)...)
+	findings = append(findings, checkInsecureRemoteInclude(p)...)
 	slices.SortStableFunc(findings, func(a, b Finding) int {
 		if c := cmp.Compare(a.File, b.File); c != 0 {
 			return c