Update design doc and implementation plan for proper LLM integration

- design-doc.md: Rewrote Section 3 (Pipeline Architecture) to describe the
  real LLM integration: dual-provider LLM client (Pi native + external),
  SHA-256 disk cache in ~/.cache/pi-project-map/, parallelization with
  4-8 concurrent requests, retries with exponential backoff, hard-error
  policy, context limit protection. Added new Section 4 with concrete
  file-level and package-level prompts.
- implementation-plan.md: Replaced old milestone schedule with current
  status and detailed M7 tasks for LLM integration: LLM client abstraction,
  external API client, Pi LLM client, disk cache, parallel batching with
  retries, rewriting llm-extract.ts, context limits, CLI/extension wiring,
  and tests.
- Minor formatting cleanup in pi-extension.ts and src/cli.ts

All 16 tests pass. TypeScript compiles clean.

pi-extension.ts

@@ -59,7 +59,11 @@ export default function (pi: ExtensionAPI) {
 		}),
 		async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
 			const targetPath = params.path || ctx.cwd;
-			const result = runCommand("init", targetPath === ctx.cwd ? [] : [targetPath], ctx.cwd);
+			const result = runCommand(
+				"init",
+				targetPath === ctx.cwd ? [] : [targetPath],
+				ctx.cwd,
+			);
 			return {
 				content: [{ type: "text", text: result.stdout || result.stderr }],
 				details: { success: result.success, cwd: ctx.cwd },
@@ -70,7 +74,8 @@ export default function (pi: ExtensionAPI) {
 	pi.registerTool({
 		name: "project_map_patch",
 		label: "Project Map Patch",
-		description: "Update .pi-map.md for the directory containing a changed file",
+		description:
+			"Update .pi-map.md for the directory containing a changed file",
 		promptSnippet: "Update project analysis after editing a source file",
 		promptGuidelines: [
 			"Use project_map_patch immediately after editing any source file",
@@ -108,10 +113,17 @@ export default function (pi: ExtensionAPI) {
 		}),
 		async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
 			const targetPath = params.path || ctx.cwd;
-			const result = runCommand("validate", targetPath === ctx.cwd ? [] : [targetPath], ctx.cwd);
+			const result = runCommand(
+				"validate",
+				targetPath === ctx.cwd ? [] : [targetPath],
+				ctx.cwd,
+			);
 			return {
 				content: [{ type: "text", text: result.stdout || result.stderr }],
-				details: { success: result.success, clean: result.stdout.includes("clean") },
+				details: {
+					success: result.success,
+					clean: result.stdout.includes("clean"),
+				},
 			};
 		},
 	});