feat(skills): add dry-run installer planning surface

src/skills/planner.ts

@@ -0,0 +1,88 @@
+import { execSync } from 'child_process';
+import type { SkillInstallerSpec } from './types.js';
+
+export interface InstallerPlanStep {
+  installerType: SkillInstallerSpec['type'];
+  command: string;
+}
+
+export interface InstallerPlanSkip {
+  installerType: SkillInstallerSpec['type'];
+  reason: string;
+}
+
+export interface InstallerPlan {
+  mode: 'dry-run';
+  steps: InstallerPlanStep[];
+  skipped: InstallerPlanSkip[];
+}
+
+export interface InstallerPlanningOptions {
+  hasBinary?: (name: string) => boolean;
+}
+
+function hasBinaryOnPath(name: string): boolean {
+  const cmd = process.platform === 'win32' ? 'where' : 'which';
+  try {
+    execSync(`${cmd} ${name}`, { stdio: 'ignore' });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+export function buildInstallerPlan(
+  installers: SkillInstallerSpec[] | undefined,
+  opts: InstallerPlanningOptions = {},
+): InstallerPlan {
+  const hasBinary = opts.hasBinary ?? hasBinaryOnPath;
+  const steps: InstallerPlanStep[] = [];
+  const skipped: InstallerPlanSkip[] = [];
+
+  for (const installer of installers ?? []) {
+    if (installer.type === 'brew') {
+      if (!hasBinary('brew')) {
+        skipped.push({ installerType: 'brew', reason: 'brew not available in PATH' });
+        continue;
+      }
+      steps.push({ installerType: 'brew', command: `brew install ${installer.packages.join(' ')}` });
+      continue;
+    }
+
+    if (installer.type === 'node') {
+      if (hasBinary('pnpm')) {
+        steps.push({ installerType: 'node', command: `pnpm add -g ${installer.packages.join(' ')}` });
+        continue;
+      }
+      if (hasBinary('npm')) {
+        steps.push({ installerType: 'node', command: `npm install -g ${installer.packages.join(' ')}` });
+        continue;
+      }
+      skipped.push({ installerType: 'node', reason: 'neither pnpm nor npm available in PATH' });
+      continue;
+    }
+
+    if (installer.type === 'go') {
+      if (!hasBinary('go')) {
+        skipped.push({ installerType: 'go', reason: 'go not available in PATH' });
+        continue;
+      }
+      for (const pkg of installer.packages) {
+        steps.push({ installerType: 'go', command: `go install ${pkg}@latest` });
+      }
+      continue;
+    }
+
+    const destination = installer.destination ?? '<default destination>';
+    steps.push({
+      installerType: 'download',
+      command: `download ${installer.url} -> ${destination}`,
+    });
+  }
+
+  return {
+    mode: 'dry-run',
+    steps,
+    skipped,
+  };
+}