feat(gh-copilot): ingest AI credit usage and add credit consumption dashboard panels

[ewega]

Summary

Resolves #8961. Adds first-class GitHub Copilot AI credit billing consumption ingestion to the gh-copilot plugin and surfaces credit-consumption KPIs on the GitHub Copilot Adoption dashboard.

The change is purely additive — there were no existing premium-request-unit (PRU) references anywhere in the plugin, grafana, or config-ui to retire, so nothing was removed.

What's included

Ingestion

• New tool-layer model _tool_copilot_ai_credit_usage (GhCopilotAiCreditUsage) with migration 20260701_add_copilot_ai_credit_usage and GetTablesInfo() registration.
• Daily collectors + extractors for the GitHub Billing Usage AI credit endpoints (API version 2026-03-10):
  • Enterprise: GET /enterprises/{enterprise}/settings/billing/ai_credit/usage
  • Organization: GET /organizations/{org}/settings/billing/ai_credit/usage (note organizations/, not orgs/)
• Reuses the existing stateful daily-iterator pattern (day-by-day year/month/day, 365-day clamp, Retry-After backoff, ignoreNoContent). Each usageItem (product/sku/model/unitType) is exploded into one row; gross/net quantity and amount stored as float64.
• 4 new subtask metas wired into register.go / subtasks.go (collect + extract × enterprise/org).

Dashboard

• New collapsible "AI Credit Consumption" row added to both mysql and postgresql github-copilot-adoption.json:
  • Stat panels: total gross credits, net credits, net amount, gross amount
  • Time series: gross vs net credits consumed over time
  • Tables: credits by model, and by product / SKU (with amount)
• Per-dialect SQL (MySQL backticks / connection_id = ${connection_id}; Postgres double-quoted identifiers, connection_id::text guard, NULLS LAST).

Tests & docs

• e2e raw + snapshot fixtures and an extended metrics_test.go exercising the org AI-credit extractor.
• Unit tests for splitDay and the pure mapAiCreditUsageReport mapping.
• Updated plugin README.md and dashboard GithubCopilotREADME.md (new subtasks, table, required billing permissions: manage_billing:copilot / admin:enterprise / admin:org, credit terminology).

Verification

• go build ./plugins/gh-copilot/... ✅
• go vet ./plugins/gh-copilot/... ✅
• gofmt -l clean ✅
• go test ./plugins/gh-copilot/tasks/... ✅

Note: backend/plugins/table_info_test.go requires generated mocks (make mock) to run in a fresh checkout — a pre-existing environment dependency unrelated to this change.