[Feature] Support multiple upstream MCP servers (named upstreams + routing)

[olivrg]

Problem

Helio v0.1 can proxy only a single upstream — upstream is a single object in the
schema, with no naming/grouping/routing layer. A beta tester hit a YAML
duplicated mapping key error trying to add a second server. (That specific error
is invalid YAML on their part — two url: keys in one mapping — but the
underlying need is real: there is no supported way to configure more than one
upstream.)

Proposed Solution

• Named upstreams (list/map instead of a single object).
• A routing layer (tool-name prefix/namespace, or explicit per-tool mapping).
• Annotation cache, audit, and policy engine keyed per upstream.
• Roadmap-sized: touches the single-forwarder assumption at packages/proxy/src/cli.ts:304-342.

Area

• Policy engine (matchers, conditions, rules)
• Audit trail
• Approvals (Slack, webhook, dashboard)
• Rate / spend limits
• Dashboard
• CLI
• Python SDK
• Configuration (helio.yaml)
• Documentation
• Other

Alternatives Considered

document explicitly that v0.1 proxies exactly one upstream.

Additional Context