chore: Make supergit multi-instance friendly

[shiroyasha]

fixes #5379

[superplanehq-integration]

👋 Commands for maintainers:

• /sp start - Start an ephemeral machine (takes ~30s)
• /sp stop - Stop a running machine (auto-executed on pr close)

[superplane-gh-integration-9000]

Docs Impact Review

This PR introduces a new per-instance environment variable (SUPERGIT_PORT) for multi-instance deployments, but multi-instance setup is not documented anywhere on the public docs site.

Suggested docs updates:

• docs.superplane.com/installation/overview — add a "Multi-instance" section (or a link to a new dedicated page) that covers the multi-instance deployment model and lists the per-instance environment variables, including SUPERGIT_PORT.
• New page (e.g. docs.superplane.com/installation/multi-instance) — document the full set of per-instance env vars from .env.multi-instance.example (ports for Public API, RabbitMQ, Postgres, Base URLs, and now SUPERGIT_PORT), explaining what each controls and how to assign distinct values across instances.

Why: The only changed file is .env.multi-instance.example, which adds SUPERGIT_PORT (ports 8080–8084) for each of the five example instances. This variable is required to avoid port conflicts when running multiple supergit service instances. The public docs have no multi-instance deployment guide at all, and this PR — which explicitly makes supergit "multi-instance friendly" — signals the feature is maturing toward operator-readiness, making the documentation gap more actionable than before.

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Posted automatically by warp-gateway · commit 70a329f