feat(compose): expose deployment context for interpolation

[defang-sam]

Description

Make the resolved deployment context available to Compose interpolation after stack selection:

• DEFANG_PROVIDER: the selected provider
• DEFANG_STACK: the selected stack name

The values are opt-in and are not injected into service environments unless referenced explicitly. Selected context takes precedence over Compose env files, while arbitrary process DEFANG_* variables remain filtered out.

Agent/MCP deployments reload Compose after interactive stack/provider selection so they use the same resolved interpolation context as direct CLI deployments. The reload preserves relative working directories, custom Compose paths, project names, and missing-config retry behavior.

This enables samples such as self-improving-mastra to pass provider and stack metadata into the deployed service without requiring portal-supplied configuration.

The canonical README now documents the interpolation behavior and regenerates the CLI/npm README copies. Also includes a formatter-only alignment correction in fixup.go required for the repository-wide lint gate.

Linked Issues

N/A

Testing

• go test -race -short ./...
• go vet ./...
• golangci-lint run
• make -B src/README.md

Checklist

• I have performed a self-review of my code
• I have added appropriate tests
• I have updated the Defang CLI docs and/or README to reflect my changes, if necessary

[lionello]

In deploy, we have the sc stack params. We get a loader from the caller. That loader did not take sc into account. Why?

[defang-sam]

You're right — this was the missing path.

The caller constructs the initial loader before SetupProvider runs, but SetupProvider may interactively select a stack, mutate sc.Stack, and load that stack's environment. So that incoming loader cannot reliably contain the final provider/stack context.

The initial LoadProject remains the pre-selection validation/discovery pass. After SetupProvider, deploy now rebuilds the loader from the resolved absolute working directory and the original project/file parameters, adds the resolved DEFANG_PROVIDER and DEFANG_STACK interpolation values, and reloads the project. That second project is what goes to ComposeUp.

I also removed the questionable shadowing: the preliminary project result is explicitly discarded, and project is declared at the final load where it is first used. The regression test covers a relative working directory and verifies that the deployed project contains the resolved provider and stack values.