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/rules.go

@@ -158,4 +158,9 @@ const (
 	// GL044: a rules:needs: entry references a job that does not exist in the pipeline.
 	// rules:needs: overrides the top-level needs: when a specific rule matches (GitLab CI 16.4+).
 	RuleRulesNeedsUnknown = "GL044"
+
+	// GL045: an include: remote: entry uses plain HTTP instead of HTTPS.
+	// CI templates fetched over HTTP are transmitted in cleartext and can be
+	// intercepted or modified in transit.
+	RuleInsecureRemoteInclude = "GL045"
 )