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/resolver/includes.go

@@ -133,6 +133,12 @@ func resolveLocalInclude(p *model.Pipeline, rawPath string, cfg fetcher.GitLabCo
 	absPath := filepath.Join(rootDir, relPath)
 	label := "local " + rawPath
 
+	// Guard against path traversal: reject any path that escapes rootDir.
+	rel, err := filepath.Rel(rootDir, absPath)
+	if err != nil || strings.HasPrefix(rel, "..") {
+		return []IncludeWarning{{Label: label, Err: fmt.Errorf("path escapes repository root: %s", rawPath)}}, nil
+	}
+
 	if visited[absPath] {
 		return nil, nil
 	}