refactor(ast-grep): split cli.ts and constants.ts into focused modules

Extract AST-grep tooling into single-responsibility files:
- cli-binary-path-resolution.ts, sg-cli-path.ts
- environment-check.ts, language-support.ts
- process-output-timeout.ts, sg-compact-json-output.ts

src/tools/ast-grep/process-output-timeout.ts

@@ -0,0 +1,28 @@
+type SpawnedProcess = {
+	stdout: ReadableStream | null
+	stderr: ReadableStream | null
+	exited: Promise<number>
+	kill: () => void
+}
+
+export async function collectProcessOutputWithTimeout(
+	process: SpawnedProcess,
+	timeoutMs: number
+): Promise<{ stdout: string; stderr: string; exitCode: number }> {
+	const timeoutPromise = new Promise<never>((_, reject) => {
+		const timeoutId = setTimeout(() => {
+			process.kill()
+			reject(new Error(`Search timeout after ${timeoutMs}ms`))
+		}, timeoutMs)
+		process.exited.then(() => clearTimeout(timeoutId))
+	})
+
+	const stdoutPromise = process.stdout ? new Response(process.stdout).text() : Promise.resolve("")
+	const stderrPromise = process.stderr ? new Response(process.stderr).text() : Promise.resolve("")
+
+	const stdout = await Promise.race([stdoutPromise, timeoutPromise])
+	const stderr = await stderrPromise
+	const exitCode = await process.exited
+
+	return { stdout, stderr, exitCode }
+}