feat: multi-instance registry and automatic token refresh on 401

[harikaduyu]

What did you do?

• Added a registry config format: a single GRAFANA_CONFIG file can now define multiple named Grafana instances under an instances key
• Single-instance format (existing) is fully backward compatible — auto-detected by the presence of url vs instances at the top level
• Added token_command field (single-instance and registry) — a shell command run via sh -c on 401; stdout becomes the new token and the request is retried once
• Registry supports a top-level token_command with {url} substitution applied per instance, overridable per instance
• MCP server: registry mode adds a required instance parameter to all four tools; a per-session client cache preserves refreshed tokens across calls
• CLI: --instance flag added to query, metrics, and instant subcommands

Why did you do it?

Two problems:

1. Token expiry caused silent failures — the MCP server read the token once at startup; after expiry every call returned 401 with no recovery. token_command + on-401 retry makes this fully automatic.

2. One-MCP-per-instance doesn't scale — with 6+ Grafana instances, each needing its own MCP server entry, context slots, and manual token management. The registry collapses N servers into one, with instance as a tool parameter.

Checklist

• Tests added/updated
• Documentation updated if needed

[github-actions]

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1 out of 2 committers have signed the CLA.
✅ harikaduyu
❌ @OpenCode

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