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/fetcher/cache.go

@@ -26,10 +26,10 @@ func cacheWrite(dir, key string, data []byte) {
 	if dir == "" {
 		return
 	}
-	if err := os.MkdirAll(dir, 0o755); err != nil {
+	if err := os.MkdirAll(dir, 0o700); err != nil {
 		return
 	}
-	_ = os.WriteFile(cachePath(dir, key), data, 0o644)
+	_ = os.WriteFile(cachePath(dir, key), data, 0o600)
 }
 
 // cachePath returns the filesystem path for a cache entry.