feat: OpenAPI 3 spec for sidecar HTTP API

[adi-suresh01]

Summary

• Adds an OpenAPI 3.1 spec for the agent HTTP API at docs/spec/openapi.yaml, generated from utoipa annotations on the route handlers and types so the spec cannot drift from the implementation.
• Adds provedex-agent --print-openapi to regenerate the spec on demand.
• provedex-core gains an openapi feature that adds ToSchema derives to AgentEvent, SignedEvent, ChainReport, ChainStatus. Default-feature build is unaffected.

The sidecar is the cross-language SDK shim per ADR 0004. Customers in Java, Go, Ruby, Kotlin, etc. now get auto-generated clients via openapi-generator, edge validation via API gateways (Kong, Apigee, AWS), and an interactive contract for procurement reviews. The four normative specs (canonical-json, event-schema-v1, signature-scheme, ledger-format) still cover the wire format that bindings sign over; OpenAPI covers the HTTP shell on top.

What changed

• Cargo.toml: add utoipa (with the yaml feature) and serde_yaml to [workspace.dependencies].
• crates/provedex-core: optional utoipa dep, new openapi feature, cfg_attr(feature = "openapi", derive(utoipa::ToSchema)) on the four public types.
• crates/provedex-agent: enables provedex-core/openapi unconditionally; ToSchema on Health and SignRequest; #[utoipa::path(...)] on each handler; new src/openapi.rs with the aggregating ApiDoc struct; new --print-openapi CLI flag that prints YAML and exits before tracing init.
• docs/spec/openapi.yaml: 338-line generated artifact, ASCII clean, regenerates deterministically.
• README links the spec under "Specs" alongside the four normative specs.
• docs/integration/sidecar.md declares the OpenAPI spec the authoritative contract; prose examples remain illustrative.

Out of scope (filed as follow-ups if needed)

• Embedded /swagger-ui/ route. Not landed in this PR; agent stays minimal.
• CI step that fails when docs/spec/openapi.yaml drifts from --print-openapi. Easy follow-up; relies on cargo run -p provedex-agent -- --print-openapi | diff -u - docs/spec/openapi.yaml.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features (core 26 unit + 4 doctest, agent 7 integration, all green)
• cargo audit clean
• cargo deny check clean
• cargo build -p provedex-core (no openapi feature) builds clean - feature gating works
• cargo run -p provedex-agent -- --print-openapi emits a valid OpenAPI 3.1 YAML doc covering 3 paths and 6 schemas
• Regeneration is deterministic (re-running --print-openapi produces an identical file)
• ASCII-only across all source + docs touched

Closes #31.