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/cli.ts

@@ -27,11 +27,11 @@ function printUsage() {
 LinkedIn Analytics CLI
 
 Usage:
-  node build/cli.js import <filename>           Import a CSV export
-  node build/cli.js report [--week W]            Generate weekly report
-  node build/cli.js report --month YYYY-MM       Generate monthly report with MoM comparison
-  node build/cli.js trends [--period P] [--metric M]  Show trends and alerts
-  node build/cli.js heatmap                           Day-of-week performance matrix
+  node --import tsx src/cli.ts import <filename>           Import a CSV export
+  node --import tsx src/cli.ts report [--week W]            Generate weekly report
+  node --import tsx src/cli.ts report --month YYYY-MM       Generate monthly report with MoM comparison
+  node --import tsx src/cli.ts trends [--period P] [--metric M]  Show trends and alerts
+  node --import tsx src/cli.ts heatmap                           Day-of-week performance matrix
 
 Options:
   --week W       ISO week (e.g., 2026-W05), defaults to current week
@@ -39,9 +39,9 @@ Options:
   --metric M     Metric to analyze: "impressions" | "reactions" | "comments" | "shares" | "clicks" | "engagementRate" (default: "impressions")
 
 Examples:
-  node build/cli.js import linkedin-export-2026-01-20.csv
-  node build/cli.js report --week 2026-W04
-  node build/cli.js trends --period quarter --metric engagementRate
+  node --import tsx src/cli.ts import linkedin-export-2026-01-20.csv
+  node --import tsx src/cli.ts report --week 2026-W04
+  node --import tsx src/cli.ts trends --period quarter --metric engagementRate
   `);
 }
 
@@ -50,7 +50,7 @@ async function handleImport(root: string, args: string[]) {
 
   if (!filename) {
     console.error("Error: Missing filename argument");
-    console.error("Usage: node build/cli.js import <filename>");
+    console.error("Usage: node --import tsx src/cli.ts import <filename>");
     process.exit(1);
   }