feat: add typed host configs for Claude, Codex, Factory, and Kiro

Extract all hardcoded host-specific values from gen-skill-docs.ts,
types.ts, preamble.ts, review.ts, and setup into typed HostConfig
objects. Each host is a single file in hosts/ with its paths, frontmatter
rules, path/tool rewrites, runtime root manifest, and install behavior.

hosts/index.ts exports all configs, derives the Host type, and provides
resolveHostArg() for CLI alias handling (e.g., 'agents' -> 'codex',
'droid' -> 'factory').

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/index.ts

@@ -0,0 +1,62 @@
+/**
+ * Host config registry.
+ *
+ * Import all host configs and derive the Host union type.
+ * Adding a new host: create hosts/myhost.ts, import here, add to ALL_HOST_CONFIGS.
+ */
+
+import type { HostConfig } from '../scripts/host-config';
+import claude from './claude';
+import codex from './codex';
+import factory from './factory';
+import kiro from './kiro';
+
+/** All registered host configs. Add new hosts here. */
+export const ALL_HOST_CONFIGS: HostConfig[] = [claude, codex, factory, kiro];
+
+/** Map from host name to config. */
+export const HOST_CONFIG_MAP: Record<string, HostConfig> = Object.fromEntries(
+  ALL_HOST_CONFIGS.map(c => [c.name, c])
+);
+
+/** Union type of all host names, derived from configs. */
+export type Host = (typeof ALL_HOST_CONFIGS)[number]['name'];
+
+/** All host names as a string array (for CLI arg validation, etc.). */
+export const ALL_HOST_NAMES: string[] = ALL_HOST_CONFIGS.map(c => c.name);
+
+/** Get a host config by name. Throws if not found. */
+export function getHostConfig(name: string): HostConfig {
+  const config = HOST_CONFIG_MAP[name];
+  if (!config) {
+    throw new Error(`Unknown host '${name}'. Valid hosts: ${ALL_HOST_NAMES.join(', ')}`);
+  }
+  return config;
+}
+
+/**
+ * Resolve a host name from a CLI argument, handling aliases.
+ * e.g., 'agents' → 'codex', 'droid' → 'factory'
+ */
+export function resolveHostArg(arg: string): string {
+  // Direct name match
+  if (HOST_CONFIG_MAP[arg]) return arg;
+
+  // Alias match
+  for (const config of ALL_HOST_CONFIGS) {
+    if (config.cliAliases?.includes(arg)) return config.name;
+  }
+
+  throw new Error(`Unknown host '${arg}'. Valid hosts: ${ALL_HOST_NAMES.join(', ')}`);
+}
+
+/**
+ * Get hosts that are NOT the primary host (Claude).
+ * These are the hosts that need generated skill docs.
+ */
+export function getExternalHosts(): HostConfig[] {
+  return ALL_HOST_CONFIGS.filter(c => c.name !== 'claude');
+}
+
+// Re-export individual configs for direct import
+export { claude, codex, factory, kiro };