Reconcile docs with MCP_GATEWAY_PORT runtime behavior and Rust guard test guidance

[Copilot]

Nightly docs reconciliation found mismatches between documented env/test behavior and implementation, centered on MCP_GATEWAY_PORT semantics/defaults and incomplete contributor guidance. This PR aligns docs and one config comment with current runtime behavior.

• Environment variable semantics (docs/ENVIRONMENT_VARIABLES.md)

  • Corrects MCP_GATEWAY_PORT wording: it is used by run.sh / run_containerized.sh to construct --listen, and by awmg only for --validate-env checks.
  • Removes conflicting value by aligning the production-table value with the script default (8000).
  • Expands the note to clearly separate script-read vars (HOST/PORT/MODE) from binary behavior.

• Contributor workflow clarity (CONTRIBUTING.md)

  • Adds a dedicated Rust Guard Tests section for make test-rust and its cargo requirement.
  • Tightens make agent-finished wording to reflect what it actually runs (go test ./... + Rust guard tests).

• Agent quick reference alignment (AGENTS.md)

  • Updates Agent-Finished description to explicitly include unit, integration, and Rust guard tests.

• JSON stdin config intent (internal/config/config_stdin.go)

  • Adds a StdinServerConfig comment clarifying intentional omission of command in JSON stdin format (stdio uses container).

`MCP_GATEWAY_PORT` is read by the binary for `--validate-env` port-mapping checks only; it does **not** auto-configure the listen address.

[Copilot]

Pull request overview

This PR reconciles project documentation (and one config struct comment) with the current runtime behavior around MCP_GATEWAY_PORT, and adds contributor guidance for running Rust guard tests so contributor workflows match what the Make targets actually do.

Changes:

• Clarifies MCP_GATEWAY_PORT semantics in docs/ENVIRONMENT_VARIABLES.md, including script vs. binary behavior and aligning the example value with the scripts’ default (8000).
• Adds a CONTRIBUTING section documenting how to run Rust guard tests via make test-rust, and refines the make agent-finished description.
• Updates the agent quick reference and adds a clarifying comment for the stdin JSON server config format (no command field).

Show a summary per file

File	Description
internal/config/config_stdin.go	Adds an explanatory comment about stdin JSON server config intentionally omitting command in favor of container.
docs/ENVIRONMENT_VARIABLES.md	Updates MCP_GATEWAY_PORT documentation to reflect script usage vs. awmg --validate-env behavior and aligns example defaults.
CONTRIBUTING.md	Documents make test-rust and refines make agent-finished wording to match actual behavior.
AGENTS.md	Updates the quick reference description of make agent-finished test coverage.

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• Files reviewed: 4/4 changed files
• Comments generated: 1

[lpcox]

@copilot address review feedback

[Copilot]

@copilot address review feedback

Addressed. I updated docs/ENVIRONMENT_VARIABLES.md so the optional table entry for MCP_GATEWAY_PORT now matches the clarified semantics (run.sh builds --listen; awmg --validate-env uses it for port-mapping checks). Included in commit c252aaa.