tsuga-cli: public HTTP API reference + dashboard-first metric discovery

[ArthurVerrez]

• Metrics: check existing dashboards before the catalog (catalog lists unemitted series; empty result usually means wrong metric, not missing data)
• New references/http-api.md: api.tsuga.com host, operation-key auth, /v1 aggregation endpoints, {data|error, requestId} envelope, unix-seconds timeRange, retention-policy lookback bounds — verified against the live API
• README: ownership & contributions section (generic skills live here; org-specific conventions go in a layered plugin)
• AGENTS.md: validation section now reflects lint CI; warn on descriptions nearing the 1024-char Codex limit
• Bump plugins to 0.7.8; realign marketplace.json version (was stuck at 0.7.4)