docs: add design proposal workflow

[jiangxt2]

Description

This PR establishes a repository-owned design proposal workflow for architectural changes that need review before implementation. It keeps proposal discussion separate from accepted ADRs, implemented architecture documentation, and support claims.

The change:

• adds design-docs/README.md to define document responsibilities, proposal entry criteria, Draft PR review, acceptance, rejection, supersession, and implementation follow-up;
• adds design-docs/template.md with motivation, goals, non-goals, current evidence, component ownership, public contracts, stability, failure and security semantics, compatibility, alternatives, acceptance criteria, open questions, decision outcome, and implementation tracking;
• adds a Tributo-specific review checklist for bounded and unbounded data boundaries, Provider and Binding extensions, ResultSink and WriteGateway ownership, AlgorithmSpec and Bundle responsibilities, model flavor loading, credential isolation, fail-closed behavior, and support evidence; and
• links the workflow from CONTRIBUTING.md and defines when a design proposal is required or unnecessary.

An accepted proposal approves a design direction but does not claim that a feature is implemented or supported. Existing ADRs, architecture documents, the decision log, Issue and PR templates, runtime code, dependencies, and package versions are unchanged.

Related issues

None.

Additional information

• uv run --locked --no-sync python scripts/pr-precheck.py passed on the final three-file documentation change, including all static layers, changed-test planning, CI parity, and Docs CI parity.
• git diff --check passed.
• No standalone pytest, Docker, or integration suite was run because this PR changes only contributor documentation and the repository precheck selected no runtime-specific test target.