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/explain.go

@@ -853,4 +853,18 @@ test:
     - if: $CI_COMMIT_BRANCH == "main"
       needs: [build, lint]`,
 	},
+
+	RuleInsecureRemoteInclude: {
+		Title:    "remote include uses plain HTTP",
+		Severity: Warning,
+		Description: "An include: remote: entry uses an http:// URL instead of https://. " +
+			"CI templates fetched over plain HTTP are transmitted in cleartext; an " +
+			"attacker with network access could intercept or modify the template " +
+			"before it is parsed. Use https:// so the connection is encrypted and " +
+			"the server's identity is verified.",
+		Example: `include:
+  - remote: http://ci-templates.example.com/build.yml`,
+		Fix: `include:
+  - remote: https://ci-templates.example.com/build.yml`,
+	},
 }

internal/linter/includes.go

@@ -0,0 +1,31 @@
+package linter
+
+import (
+	"fmt"
+	"strings"
+
+	"git.k3nny.fr/glint/internal/model"
+)
+
+// checkInsecureRemoteInclude warns when an include: remote: entry uses plain
+// HTTP (GL045). The file is still fetched and linted; the finding is a warning
+// so pipelines that use HTTP for internal infra are not blocked.
+func checkInsecureRemoteInclude(p *model.Pipeline) []Finding {
+	var findings []Finding
+	for _, inc := range p.Include {
+		m, ok := inc.(map[string]any)
+		if !ok {
+			continue
+		}
+		remote, _ := m["remote"].(string)
+		if strings.HasPrefix(remote, "http://") {
+			findings = append(findings, Finding{
+				Severity: Warning,
+				Rule:     RuleInsecureRemoteInclude,
+				File:     p.SourceFile,
+				Message:  fmt.Sprintf("remote include %q uses plain HTTP; CI templates are fetched unencrypted — prefer HTTPS", remote),
+			})
+		}
+	}
+	return findings
+}

internal/linter/includes_test.go

@@ -0,0 +1,73 @@
+package linter
+
+import (
+	"testing"
+
+	"git.k3nny.fr/glint/internal/model"
+)
+
+func TestCheckInsecureRemoteInclude(t *testing.T) {
+	tests := []struct {
+		name    string
+		include []any
+		wantGL  string // expected rule ID, empty = no findings
+	}{
+		{
+			name:    "http URL triggers GL045",
+			include: []any{map[string]any{"remote": "http://example.com/ci.yml"}},
+			wantGL:  RuleInsecureRemoteInclude,
+		},
+		{
+			name:    "https URL is clean",
+			include: []any{map[string]any{"remote": "https://example.com/ci.yml"}},
+		},
+		{
+			name:    "local include ignored",
+			include: []any{map[string]any{"local": "/templates/ci.yml"}},
+		},
+		{
+			name:    "string include ignored",
+			include: []any{"/templates/ci.yml"},
+		},
+		{
+			name:    "no includes",
+			include: nil,
+		},
+		{
+			name: "mixed: http fires, https does not",
+			include: []any{
+				map[string]any{"remote": "https://ok.example.com/ci.yml"},
+				map[string]any{"remote": "http://bad.example.com/ci.yml"},
+			},
+			wantGL: RuleInsecureRemoteInclude,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			p := &model.Pipeline{
+				Include: tc.include,
+				Jobs:    map[string]model.Job{},
+			}
+			findings := checkInsecureRemoteInclude(p)
+			if tc.wantGL == "" {
+				if len(findings) != 0 {
+					t.Errorf("expected no findings, got: %v", findings)
+				}
+				return
+			}
+			found := false
+			for _, f := range findings {
+				if f.Rule == tc.wantGL {
+					found = true
+					if f.Severity != Warning {
+						t.Errorf("severity = %v; want Warning", f.Severity)
+					}
+				}
+			}
+			if !found {
+				t.Errorf("expected finding %s, got: %v", tc.wantGL, findings)
+			}
+		})
+	}
+}

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

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"
 )