fix(skills): cache GitHub repo trees to avoid rate-limit exhaustion on install

Skills.sh installs hit the GitHub API 45 times per install because the
same repo tree was fetched 6 times redundantly. Combined with search
(23 API calls), this totals 68 — exceeding the unauthenticated rate
limit of 60 req/hr, causing 'Could not fetch' errors for users without
a GITHUB_TOKEN.

Changes:
- Add _get_repo_tree() cache to GitHubSource — repo info + recursive
  tree fetched once per repo per source instance, eliminating 10
  redundant API calls (6 tree + 4 candidate 404s)
- _download_directory_via_tree returns {} (not None) when cached tree
  shows path doesn't exist, skipping unnecessary Contents API fallback
- _check_rate_limit_response() detects exhausted quota and sets
  is_rate_limited flag
- do_install() shows actionable hint when rate limited: set
  GITHUB_TOKEN or install gh CLI

Before: 45 API calls per install (68 total with search)
After:  31 API calls per install (54 total with search — under 60/hr)

Reported by community user from Vietnam (no GitHub auth configured).

hermes_cli/skills_hub.py

@@ -335,7 +335,23 @@ def do_install(identifier: str, category: str = "", force: bool = False,
     meta, bundle, _matched_source = _resolve_source_meta_and_bundle(identifier, sources)
 
     if not bundle:
-        c.print(f"[bold red]Error:[/] Could not fetch '{identifier}' from any source.\n")
+        # Check if any source hit GitHub API rate limit
+        rate_limited = any(
+            getattr(src, "is_rate_limited", False)
+            or getattr(getattr(src, "github", None), "is_rate_limited", False)
+            for src in sources
+        )
+        c.print(f"[bold red]Error:[/] Could not fetch '{identifier}' from any source.")
+        if rate_limited:
+            c.print(
+                "[yellow]Hint:[/] GitHub API rate limit exhausted "
+                "(unauthenticated: 60 requests/hour).\n"
+                "Set [bold]GITHUB_TOKEN[/] in your .env or install the "
+                "[bold]gh[/] CLI and run [bold]gh auth login[/] "
+                "to raise the limit to 5,000/hr.\n"
+            )
+        else:
+            c.print()
         return
 
     # Auto-detect category for official skills (e.g. "official/autonomous-ai-agents/blackbox")