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/config/config.go

@@ -38,6 +38,12 @@ type Config struct {
 	// CacheDir is the default directory for caching fetched remote includes.
 	// Overridden by the --cache-dir flag.
 	CacheDir string `yaml:"cache_dir"`
+
+	// Proxy is the HTTP proxy URL for fetching remote includes and GitLab API
+	// calls (e.g. "http://proxy.example.com:8080"). Overridden by the --proxy
+	// flag. When empty, system proxy settings (HTTP_PROXY / HTTPS_PROXY /
+	// NO_PROXY env vars) are used automatically.
+	Proxy string `yaml:"proxy"`
 }
 
 // Load searches for a .glint.yml file starting from dir and walking up toward