fix: don't fallback to system 'sg' command for ast-grep

On Linux systems, 'sg' is a mailutils command, not ast-grep. The previous
fallback would silently run the wrong binary when ast-grep wasn't found.

Changes:
- getSgCliPath() now returns string | null instead of string
- Fallback changed from 'sg' to null
- Call sites now check for null and return user-facing error with
  installation instructions
- checkEnvironment() updated to handle null path

Fixes #1365

src/agents/dynamic-agent-prompt-builder.ts

@@ -421,7 +421,7 @@ export function buildUltraworkSection(
 
     lines.push("**Agents** (for specialized consultation/exploration):")
     for (const agent of sortedAgents) {
-      const shortDesc = agent.description.split(".")[0] || agent.description
+      const shortDesc = agent.description.length > 120 ? agent.description.slice(0, 120) + "..." : agent.description
       const suffix = agent.name === "explore" || agent.name === "librarian" ? " (multiple)" : ""
       lines.push(`- \`${agent.name}${suffix}\`: ${shortDesc}`)
     }