feat(ast-grep): detect regex-style pattern misuse before returning empty results

LLMs frequently call ast_grep_search with regex-style patterns like
'func.*build|BuildMode|projectReferences' instead of AST patterns. The
search silently returns zero matches with no useful feedback, so the
model retries with a different regex-shaped pattern and loops.

Extract hint generation into pattern-hints.ts and add detectors for the
four dominant misuse modes:
- regex escapes (\\w, \\d, \\s, \\b)
- character-class ranges ([a-z], [0-9])
- regex wildcards (.* .+) with no meta-vars
- pure alternation (foo|bar|baz with no structural syntax)

Heuristics are designed to be safe on valid AST patterns: bitwise OR
'$A | $B' and Rust closures '|x| x + 1' are not flagged. Language-
specific shape hints (trailing-colon Python, body-less JS/TS/Go/Rust
functions) are preserved and extended to Go and Rust.

src/tools/ast-grep/tools.ts

@@ -3,6 +3,7 @@ import { tool, type ToolDefinition } from "@opencode-ai/plugin/tool"
 import { CLI_LANGUAGES } from "./constants"
 import { runSg } from "./cli"
 import { formatSearchResult, formatReplaceResult } from "./result-formatter"
+import { getPatternHint } from "./pattern-hints"
 import type { CliLanguage } from "./types"
 
 async function showOutputToUser(context: unknown, output: string): Promise<void> {
@@ -12,29 +13,6 @@ async function showOutputToUser(context: unknown, output: string): Promise<void>
   await ctx.metadata?.({ metadata: { output } })
 }
 
-function getEmptyResultHint(pattern: string, lang: CliLanguage): string | null {
-  const src = pattern.trim()
-
-  if (lang === "python") {
-    if (src.startsWith("class ") && src.endsWith(":")) {
-      const withoutColon = src.slice(0, -1)
-      return `Hint: Remove trailing colon. Try: "${withoutColon}"`
-    }
-    if ((src.startsWith("def ") || src.startsWith("async def ")) && src.endsWith(":")) {
-      const withoutColon = src.slice(0, -1)
-      return `Hint: Remove trailing colon. Try: "${withoutColon}"`
-    }
-  }
-
-  if (["javascript", "typescript", "tsx"].includes(lang)) {
-    if (/^(export\s+)?(async\s+)?function\s+\$[A-Z_]+\s*$/i.test(src)) {
-      return `Hint: Function patterns need params and body. Try "function $NAME($$$) { $$$ }"`
-    }
-  }
-
-  return null
-}
-
 export function createAstGrepTools(ctx: PluginInput): Record<string, ToolDefinition> {
   const ast_grep_search: ToolDefinition = tool({
     description:
@@ -63,7 +41,7 @@ export function createAstGrepTools(ctx: PluginInput): Record<string, ToolDefinit
         let output = formatSearchResult(result)
 
         if (result.matches.length === 0 && !result.error) {
-          const hint = getEmptyResultHint(args.pattern, args.lang as CliLanguage)
+          const hint = getPatternHint(args.pattern, args.lang as CliLanguage)
           if (hint) {
             output += `\n\n${hint}`
           }