chore: add structured GitHub issue templates

[VPRamon]

Summary

Add structured GitHub Issue Forms for the four issue types used by the project:

• Bug — defects/regressions with reproduction, impact, evidence, and a regression-verification contract.
• Feature — new product capabilities with explicit user value, spod/Siderust ownership, inputs/outputs, acceptance criteria, and verification.
• Documentation — documentation work tied to an explicit source of truth and reviewable acceptance criteria.
• Product use case — an end-to-end living specification that serves simultaneously as product documentation, functional contract, and verification plan.

Blank issues are disabled so new work starts with enough context to triage correctly.

Use cases as specifications

The use-case form explicitly captures:

• actor/customer and problem;
• product status (core, extension, reference/example, discovery);
• trigger/preconditions;
• input contract;
• end-to-end workflow;
• spod responsibilities;
• Siderust/upstream responsibilities;
• output/artifact/API contract;
• correctness and operational quality gates;
• failure behavior;
• acceptance criteria;
• verification evidence;
• observability/supportability;
• dependencies and non-goals;
• documentation deliverables.

This is intended to let a use-case issue survive implementation as useful product documentation and as a regression/verification contract.

Product boundary

The feature/use-case templates reinforce the current architecture:

• reusable scientific/numerical/format functionality belongs in Siderust;
• workflows, orchestration, policy, interfaces, artifacts, provenance, deployment, and product-level verification belong in spod;
• missing required upstream semantics should be identified explicitly rather than silently reimplemented or downgraded.

Documentation

Add docs/contributing/issues.md describing:

• when to use each template;
• how issues fit the product-development lifecycle;
• use cases as living specifications;
• good acceptance criteria;
• required verification evidence;
• closing criteria.

CONTRIBUTING.md now links to these guidelines.

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