feat: add glab (GitLab CLI) proxy support

Add token-optimized filtering for GitLab CLI (glab) commands:

- `glab mr list`: Compact MR listing with state/draft icons (~85% savings)
- `glab mr view`: Essential MR details with filtered description (~80% savings)
- `glab issue list`: Compact issue listing (~80% savings)
- `glab issue view`: Essential issue details (~75% savings)
- All other subcommands (api, pipeline, release): passthrough with tracking

Follows the established gh_cmd.rs pattern:
- JSON output parsing with serde_json
- State icons (ultra_compact mode support)
- Description filtering (HTML comments, images, horizontal rules)
- Identifier extraction for view commands
- Graceful fallback to raw command on filter failure
- Exit code propagation

Includes:
- Test fixtures with sanitized data
- Unit tests for all filter functions
- Token savings assertions (>=60%)
- Discover rules for hook rewrite integration

Closes #851

Author: Smasherr

src/discover/rules.rs

@@ -15,6 +15,7 @@ pub struct RtkRule {
 pub const PATTERNS: &[&str] = &[
     r"^git\s+(?:-[Cc]\s+\S+\s+)*(status|log|diff|show|add|commit|push|pull|branch|fetch|stash|worktree)",
     r"^gh\s+(pr|issue|run|repo|api|release)",
+    r"^glab\s+(mr|issue|api|pipeline|release|repo)",
     r"^cargo\s+(build|test|clippy|check|fmt|install)",
     r"^pnpm\s+(list|ls|outdated|install)",
     r"^npm\s+(run|exec)",
@@ -110,6 +111,14 @@ pub const RULES: &[RtkRule] = &[
         subcmd_savings: &[("pr", 87.0), ("run", 82.0), ("issue", 80.0)],
         subcmd_status: &[],
     },
+    RtkRule {
+        rtk_cmd: "rtk glab",
+        rewrite_prefixes: &["glab"],
+        category: "GitLab",
+        savings_pct: 82.0,
+        subcmd_savings: &[("mr", 85.0), ("issue", 80.0)],
+        subcmd_status: &[],
+    },
     RtkRule {
         rtk_cmd: "rtk cargo",
         rewrite_prefixes: &["cargo"],