Freeze portable Content Scout V1 contracts

[ferueda]

Summary

• freeze the language-neutral V1 execution request and response schemas
• freeze Content Scout input and candidate schemas with shared fixtures
• add strict Go domain validation and stable job, configuration, and artifact identities
• include the final sanitized-text byte provenance fields and schema digest

Review path

1. Start with internal/application/agent_execution.go for the generic executor boundary.
2. Read internal/domain/agent.go for job, execution, receipt, and run contracts.
3. Read internal/domain/artifact.go for Content Scout input, candidates, and artifact identity.
4. Use contracts and internal/application/agent_contract_schema_test.go to verify the wire format.

Boundaries

Contracts only. This PR contains no SQLite store, migration, queue implementation, agent implementation, or compatibility path.

Verification

• make check on the rebuilt slice

Stack

Based on #20. Merge and reconcile #20 first.

[ferueda]

Superseded by #20. Noema no longer owns portable agent execution, job, run, or artifact contracts. The branch is retained for reference; any useful Content Scout output-schema ideas will be reconsidered in the independent consumer project.