feat(zeroshot-rust): define independent issue and source-code provider contracts

[tomdps]

Implements #694.

What changed

• Adds separate bounded IssueProvider and SourceCodeProvider contracts, registries, identities, capabilities, and evidence types.
• Enforces inspect-before-repeat and merge-receipt binding to the requested base/head.
• Adds deterministic contract, bounds, architecture, and compile-fail coverage.
• Isolates the existing Node unit-test environment so the full repository gate remains reliable.
• Documents the new product-private provider boundaries in AGENTS.md.

Validation

• cargo build -p zeroshot-rust --all-targets
• cargo test -p zeroshot-rust --test provider_contracts
• cargo test -p zeroshot-rust --test provider_bounds
• cargo test -p zeroshot-rust --test backend_boundary
• cargo test -p zeroshot-rust --test architecture
• cargo test -p zeroshot-rust --doc
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• npm run protocol:check
• npm run lint
• npm run test:unit
• opcore check --changed

Both Zeroshot validation agents approved the final commit.