feat(openspec): Phase 4 Developer Growth & Go-to-Market Readiness

OpenSpec change: phase-4-developer-growth (spec-driven, 4/4 artifacts)

6 workstreams, 90 implementation tasks, delivery sequence:
WS1 → WS2 + WS3 (parallel) → WS4 → WS5 → WS6

Workstreams:
1. Production Hardening — ioredis rate limiting, DB pool tuning, /health/detailed, k6 load tests
2. Developer Portal — Next.js 14, Swagger UI explorer, onboarding wizard, pricing/SDK pages
3. CLI Tool — sentryagent npm CLI, 5 commands, shell completion
4. Agent Marketplace — public searchable registry powered by existing agent/DID infrastructure
5. GitHub Actions — register-agent + issue-token Actions via OIDC (no stored secrets)
6. Billing & Usage Metering — Stripe Checkout, webhook-driven state, free tier enforcement

New capabilities (8 specs): production-hardening, developer-portal, cli-tool,
agent-marketplace, github-actions, billing-metering (+delta: web-dashboard, monitoring)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

openspec/changes/phase-4-developer-growth/specs/web-dashboard/spec.md

@@ -0,0 +1,23 @@
+## ADDED Requirements
+
+### Requirement: Usage dashboard tab displays per-tenant metering data
+The web dashboard SHALL include a "Usage" tab in the main navigation displaying the current billing period's usage: API calls used / daily limit, active agents count / agent limit, token issuances this period. Usage data SHALL be fetched from `GET /billing/usage` (new authenticated endpoint). The tab SHALL update on page load and on a 60-second polling interval.
+
+#### Scenario: Usage tab shows current metrics
+- **WHEN** an authenticated user navigates to the Usage tab
+- **THEN** the dashboard displays current API call count, agent count, and token issuance count for the current billing period
+
+#### Scenario: Free tier warning shown when approaching limit
+- **WHEN** a free-tier tenant has used ≥ 80% of their daily API call limit
+- **THEN** a warning banner is displayed with a link to the upgrade/pricing page
+
+### Requirement: Billing status panel shows subscription tier and upgrade CTA
+The web dashboard Usage tab SHALL include a billing status panel showing: current tier (Free / Paid), subscription status (active / cancelled / trial), and — for free-tier tenants — an "Upgrade" button linking to `POST /billing/checkout` flow.
+
+#### Scenario: Free tier tenant sees upgrade CTA
+- **WHEN** a free-tier tenant views the Usage tab
+- **THEN** an "Upgrade to Paid" button is visible and initiates Stripe Checkout when clicked
+
+#### Scenario: Paid tier tenant sees subscription status
+- **WHEN** a paid-tier tenant views the Usage tab
+- **THEN** the panel shows "Paid" tier with subscription status and next renewal date, with no upgrade CTA