feat(cli): add Bun single executable binary support

Enables building hapi as standalone Bun-compiled executables for macOS,
Linux, and Windows (x64/arm64). Adds build script, bootstrap entry point,
runtime asset management, and automatic deployment of bundled tools
(ripgrep, difftastic). Includes MCP stdio bridge support and proper
environment handling for compiled binaries. Updates documentation with
build and installation instructions for single executable distribution.

cli/src/utils/bunRuntime.ts

@@ -0,0 +1,14 @@
+export function withBunRuntimeEnv(env: NodeJS.ProcessEnv = process.env): NodeJS.ProcessEnv {
+    const bunRuntime = (globalThis as typeof globalThis & { Bun?: { isCompiled?: boolean } }).Bun;
+    const argv1 = process.argv[1] ?? '';
+    const isCompiled = Boolean(bunRuntime?.isCompiled) || argv1.includes('$bunfs');
+
+    if (!isCompiled) {
+        return env;
+    }
+
+    return {
+        ...env,
+        BUN_BE_BUN: '1'
+    };
+}

cli/src/utils/spawnHappyCLI.ts

@@ -55,7 +55,7 @@
 
 import { spawn, SpawnOptions, type ChildProcess } from 'child_process';
 import { join } from 'node:path';
-import { projectPath } from '@/projectPath';
+import { isBunCompiled, projectPath } from '@/projectPath';
 import { logger } from '@/ui/logger';
 import { existsSync } from 'node:fs';
 
@@ -84,12 +84,39 @@ function resolveEntrypointForBun(projectRoot: string): string {
   throw new Error('No CLI entrypoint found for Bun runtime (expected dist/index.mjs or src/index.ts)');
 }
 
-export function spawnHappyCLI(args: string[], options: SpawnOptions = {}): ChildProcess {
+export interface HappyCliCommand {
+  command: string;
+  args: string[];
+}
+
+export function getHappyCliCommand(args: string[]): HappyCliCommand {
+  if (isBunCompiled()) {
+    return {
+      command: process.execPath,
+      args
+    };
+  }
+
   const projectRoot = projectPath();
   const distEntrypoint = join(projectRoot, 'dist', 'index.mjs');
   const isBunRuntime = Boolean((process.versions as Record<string, string | undefined>).bun);
   const entrypoint = isBunRuntime ? resolveEntrypointForBun(projectRoot) : distEntrypoint;
 
+  const spawnArgs = isBunRuntime ? [entrypoint, ...args] : [
+    '--no-warnings',
+    '--no-deprecation',
+    entrypoint,
+    ...args
+  ];
+
+  return {
+    command: process.execPath,
+    args: spawnArgs
+  };
+}
+
+export function spawnHappyCLI(args: string[], options: SpawnOptions = {}): ChildProcess {
+
   let directory: string | URL | undefined;
   if ('cwd' in options) {
     directory = options.cwd
@@ -104,19 +131,16 @@ export function spawnHappyCLI(args: string[], options: SpawnOptions = {}): Child
   const fullCommand = `hapi ${args.join(' ')}`;
   logger.debug(`[SPAWN HAPI CLI] Spawning: ${fullCommand} in ${directory}`);
   
-  const spawnCommand = process.execPath;
-  const spawnArgs = isBunRuntime ? [entrypoint, ...args] : [
-    '--no-warnings',
-    '--no-deprecation',
-    entrypoint,
-    ...args
-  ];
+  const { command: spawnCommand, args: spawnArgs } = getHappyCliCommand(args);
 
   // Sanity check of the entrypoint path exists
-  if (!existsSync(entrypoint)) {
-    const errorMessage = `Entrypoint ${entrypoint} does not exist`;
-    logger.debug(`[SPAWN HAPI CLI] ${errorMessage}`);
-    throw new Error(errorMessage);
+  if (!isBunCompiled()) {
+    const entrypoint = spawnArgs.find((arg) => arg.endsWith('index.mjs') || arg.endsWith('index.ts'));
+    if (entrypoint && !existsSync(entrypoint)) {
+      const errorMessage = `Entrypoint ${entrypoint} does not exist`;
+      logger.debug(`[SPAWN HAPI CLI] ${errorMessage}`);
+      throw new Error(errorMessage);
+    }
   }
   
   return spawn(spawnCommand, spawnArgs, options);