Add remote script loader with intelligent completions

Features:
- Remote script execution with version control (name@ref)
- Background daemon for fast completions (< 15ms)
- HTTP caching with ETag/Last-Modified support
- Context-aware completions with async support
- Comprehensive documentation and type definitions

Changes:
- Add loader.ts with daemon mode
- Add completions.d.ts type definitions
- Add COMPLETIONS.md guide for developers
- Add README.md with usage examples
- Update configure_ssh_key.ts with async completions
- Use JSR imports for remote execution compatibility

completions.d.ts

@@ -0,0 +1,149 @@
+/**
+ * Type definitions for script completion functions
+ * 
+ * Scripts can export a `getCompletions` function to provide intelligent
+ * shell completion suggestions for their arguments.
+ * 
+ * @example
+ * ```typescript
+ * export function getCompletions(ctx: CompletionContext): string[] {
+ *   const { current, args } = ctx;
+ *   
+ *   // Suggest values for --key flag
+ *   if (current.startsWith("--key=")) {
+ *     return ["--key=~/.ssh/id_rsa", "--key=~/.ssh/id_ed25519"];
+ *   }
+ *   
+ *   // Check what's already provided
+ *   const hasForce = args.includes("--force");
+ *   
+ *   // Return available flags
+ *   return ["--force", "--dry-run", "--help"].filter(
+ *     flag => flag !== "--force" || !hasForce
+ *   );
+ * }
+ * ```
+ */
+
+/**
+ * Context provided to the completion function
+ */
+export interface CompletionContext {
+  /**
+   * All arguments provided so far (excluding the script name)
+   * 
+   * @example
+   * If user typed: `scripts my-script --flag1 value1 --flag2`
+   * Then args would be: ["--flag1", "value1", "--flag2"]
+   */
+  args: string[];
+
+  /**
+   * The current word being completed
+   * 
+   * @example
+   * - User typed `--k` and pressed TAB → current = "--k"
+   * - User typed `--key=` and pressed TAB → current = "--key="
+   * - User typed `--port=22` and pressed TAB → current = "--port=22"
+   */
+  current: string;
+
+  /**
+   * Position of the current word in the args array (0-indexed)
+   * 
+   * @example
+   * If user typed: `scripts my-script arg1 arg2 <TAB>`
+   * Then position would be 2 (0=arg1, 1=arg2, 2=current)
+   */
+  position: number;
+}
+
+/**
+ * Completion function that scripts should export
+ * 
+ * This function is called by the loader daemon when the user requests
+ * shell completions. It should return an array of completion suggestions
+ * based on the provided context.
+ * 
+ * Can be synchronous or asynchronous (return Promise<string[]>).
+ * 
+ * @param ctx - Completion context with current state
+ * @returns Array of completion suggestions (can be empty), or Promise resolving to array
+ * 
+ * @remarks
+ * - Return empty array if no completions are available
+ * - Suggestions are filtered by the shell, so return all matches
+ * - Can include partial matches (e.g., "--key=" to prompt for value)
+ * - Keep execution fast (< 50ms recommended) for responsive UX
+ * - Use Deno APIs to discover filesystem, environment, etc.
+ * - Async functions enable API calls, but keep them fast
+ * 
+ * @example Simple flag completions
+ * ```typescript
+ * export function getCompletions(ctx: CompletionContext): string[] {
+ *   return ["--verbose", "--quiet", "--help"];
+ * }
+ * ```
+ * 
+ * @example Async completions with API call
+ * ```typescript
+ * export async function getCompletions(ctx: CompletionContext): Promise<string[]> {
+ *   const { current } = ctx;
+ *   
+ *   if (current.startsWith("--branch=")) {
+ *     try {
+ *       const response = await fetch("https://api.github.com/repos/user/repo/branches");
+ *       const branches = await response.json();
+ *       return branches.map(b => `--branch=${b.name}`);
+ *     } catch {
+ *       return ["--branch=main"];
+ *     }
+ *   }
+ *   
+ *   return ["--branch=", "--help"];
+ * }
+ * ```
+ * 
+ * @example Context-aware completions
+ * ```typescript
+ * export function getCompletions(ctx: CompletionContext): string[] {
+ *   const { current, args } = ctx;
+ *   
+ *   // Complete values for flags
+ *   if (current.startsWith("--output=")) {
+ *     return ["--output=json", "--output=yaml", "--output=text"];
+ *   }
+ *   
+ *   // Avoid duplicate flags
+ *   const usedFlags = new Set(args.map(a => a.split("=")[0]));
+ *   
+ *   const allFlags = ["--output=", "--verbose", "--format="];
+ *   return allFlags.filter(f => !usedFlags.has(f.replace("=", "")));
+ * }
+ * ```
+ * 
+ * @example Dynamic completions with filesystem
+ * ```typescript
+ * export function getCompletions(ctx: CompletionContext): string[] {
+ *   const { current } = ctx;
+ *   
+ *   if (current.startsWith("--config=")) {
+ *     try {
+ *       // Find config files in current directory
+ *       const files: string[] = [];
+ *       for (const entry of Deno.readDirSync(".")) {
+ *         if (entry.isFile && entry.name.endsWith(".json")) {
+ *           files.push(`--config=${entry.name}`);
+ *         }
+ *       }
+ *       return files;
+ *     } catch {
+ *       return ["--config=config.json"];
+ *     }
+ *   }
+ *   
+ *   return ["--config=", "--help"];
+ * }
+ * ```
+ */
+export type CompletionFunction = (ctx: CompletionContext) => string[] | Promise<string[]>;