fix(linkedin-studio): anchor analytics root on plugin marker + surface npm install

Wave 2 / Step 4 of the remediation plan (docs/remediation/plan.md).

PRIMARY (the real fresh-clone failure):
- scripts/analytics/node_modules is gitignored, so a fresh clone has neither
  tsx nor csv-parse. Surface an idempotent `npm install --silent` prerequisite
  at point-of-use in report.md (Step 1b) and import.md (Step 4).

DEVIATION FROM PLAN (correction-in-scope, to satisfy the plan's own Verify gate):
- The plan assumed prepending `npm install` was sufficient. Verified it is NOT:
  the commands invoke the CLI with an absolute src/cli.ts path but from the
  user's arbitrary CWD, and `node --import tsx` resolves the `tsx` specifier
  relative to CWD, not the script. There is no global tsx, so the call still
  fails with ERR_MODULE_NOT_FOUND from any CWD other than scripts/analytics.
- Complete fix: invoke the locally-installed tsx by its absolute
  node_modules/.bin/tsx path in all CLI calls (report.md x10, import.md x3), so
  they resolve from any working directory once the install above has run.
  Verified: 0 ERR_MODULE_NOT_FOUND running `report` from /tmp.

SECONDARY (latent correctness / hardening):
- Add findPluginRoot(): walks up to the dir holding .claude-plugin/plugin.json
  and anchors getAnalyticsRoot() on it, falling back to the legacy 4-up count.
  MEASURED that ../../../../ already resolved to the plugin root from BOTH
  src/utils and build/utils (both 4 levels deep), so the plan's "src-vs-build
  depth miscalibration" premise was false — this is correct-by-construction
  hardening (survives a future source move), not a live-bug fix.
- Reconcile cli.ts usage/help text: `node build/cli.js` -> `node --import tsx
  src/cli.ts` (the real runtime).
- Fix report.md troubleshooting: "Verify tsx is available" -> the actual
  install command on ERR_MODULE_NOT_FOUND.

Test-first: scripts/analytics/tests/storage-root.test.ts (red on missing
findPluginRoot export, green after). Full suite 106/106, tsc --noEmit clean,
structural lint 0 failed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/analytics/src/utils/storage.ts

@@ -6,18 +6,46 @@ import type { AnalyticsBatch, WeeklyReport, MonthlyReport, PostAnalytics } from
 const __dirname = dirname(fileURLToPath(import.meta.url));
 
 /**
- * Get the analytics root directory from environment or default location
- * Default is assets/analytics relative to the plugin root
+ * Walk up from `startDir` until a directory containing `.claude-plugin/plugin.json`
+ * is found. Returns that directory, or null if no marker is found before the
+ * filesystem root.
+ *
+ * Anchoring on the plugin marker keeps the analytics root correct by
+ * construction regardless of whether this module runs from `src/utils/`
+ * (under tsx) or `build/utils/` (compiled) — and survives a future source
+ * move that a hardcoded "../../../../" count would silently break.
+ */
+export function findPluginRoot(startDir: string): string | null {
+  let dir = resolve(startDir);
+
+  // dirname() of the filesystem root returns the root itself; stop when it
+  // no longer changes.
+  for (;;) {
+    if (existsSync(join(dir, ".claude-plugin", "plugin.json"))) {
+      return dir;
+    }
+    const parent = dirname(dir);
+    if (parent === dir) {
+      return null;
+    }
+    dir = parent;
+  }
+}
+
+/**
+ * Get the analytics root directory from environment or default location.
+ * Default is assets/analytics under the plugin root (the dir holding
+ * .claude-plugin/plugin.json). The ANALYTICS_ROOT env override is the test seam.
  */
 export function getAnalyticsRoot(): string {
   if (process.env.ANALYTICS_ROOT) {
     return resolve(process.env.ANALYTICS_ROOT);
   }
 
-  // Build output is at: scripts/analytics/build/utils/storage.js
-  // Plugin root is 4 levels up: ../../../../
-  // Then assets/analytics from there
-  const pluginRoot = resolve(__dirname, "../../../../");
+  // Anchor on the .claude-plugin/plugin.json marker. Fall back to the legacy
+  // 4-levels-up count (scripts/analytics/{src,build}/utils -> plugin root) only
+  // if no marker is found (e.g. an unusual extraction without the manifest).
+  const pluginRoot = findPluginRoot(__dirname) ?? resolve(__dirname, "../../../../");
   return join(pluginRoot, "assets", "analytics");
 }