feat(core): add a configuration system with file and environment support #7

Problem

There is no configuration system; networks, endpoints, and output settings cannot be persisted.

Why this matters

Configuration is required for real workflows and CI.

Context

devkit-core currently exposes only types.

Scope

• Load configuration from a TOML file and STELLAR_FOUNDRY_* environment variables.

• Validate unknown fields and referenced values.

Out of scope

• Remote configuration.

Technical guidance

• Treat configuration files as untrusted input; never let them enable network access implicitly.

Relevant modules/files

• crates/devkit-core/src/lib.rs

Acceptance criteria

• Config precedence is documented and tested

• Invalid config is rejected with a clear error

Tests required

• Parsing, precedence, and rejection tests

Security considerations

Configuration must never contain or log secrets; document this.

Performance considerations

None specific.

Documentation requirements

Add docs/CONFIGURATION.md.

Dependencies

None.

Definition of done

Merged with tests passing and CI green.

Complexity recommendation

difficulty/medium

Contribution guidelines

• Assignment required before starting. Request assignment on this issue; do not open a pull request until a maintainer has assigned you.

• Your pull request must include Closes #7.

• Run the local gate before opening the PR:

  cargo fmt --all -- --check
  cargo clippy --all-targets --all-features -- -D warnings
  cargo test --all
  

• Example commit message: feat(core): add a configuration system with file and environment support

• Suggested timeframe: complete within the active Wave cycle.

• Complexity is tagged with a difficulty/* label. Drips determines the applicable points and budget through its own system.

• See docs/WAVE.md and CONTRIBUTING.md.