feat: first-run activation — project-aware scaffold, router front door, onboarding nudges

Adds the activation system that drives a new install toward a concrete first move:
- bin/gstack-first-task-detect: local-git+filesystem repo classifier emitting one
  validated enum bucket (greenfield/code_<lang>/branch_ahead/dirty_default/clean_default),
  portable timeouts, fail-safe empty output.
- generate-first-run-guidance.ts: unified preamble section — first-run project-aware
  scaffold + returning-session plan->review->ship tip, gated on a persistent .activated
  marker and never run in headless. Detection wired lazily in generate-preamble-bash.ts.
- SKILL.md.tmpl: top-level gstack skill is now a pure router (browse body removed; it
  lives in /browse), routing any request and sending browser/QA work to /browse.
- setup: first-move nudge on first install. office-hours: closing handoff that launches
  the next review via the Skill tool.
- telemetry-ingest: accept onboarding/first_task_scaffold_shown/handoff/route event types.

scripts/resolvers/preamble.ts

@@ -32,6 +32,7 @@ import {
 import { generateLakeIntro } from './preamble/generate-lake-intro';
 import { generateTelemetryPrompt } from './preamble/generate-telemetry-prompt';
 import { generateProactivePrompt } from './preamble/generate-proactive-prompt';
+import { generateFirstRunGuidance } from './preamble/generate-first-run-guidance';
 import { generateRoutingInjection } from './preamble/generate-routing-injection';
 import { generateVendoringDeprecation } from './preamble/generate-vendoring-deprecation';
 import { generateSpawnedSessionCheck } from './preamble/generate-spawned-session-check';
@@ -94,6 +95,7 @@ export function generatePreamble(ctx: TemplateContext): string {
     generateLakeIntro(),
     generateTelemetryPrompt(ctx),
     generateProactivePrompt(ctx),
+    generateFirstRunGuidance(ctx),
     generateRoutingInjection(ctx),
     generateVendoringDeprecation(ctx),
     generateSpawnedSessionCheck(),