feat: add tenant-scoped artifact uploads

[Notyet1307]

Change

• Add POST /v1/runs/{runID}/artifacts?name=... for bounded, streaming raw-byte uploads.
• Compute SHA-256 and optionally verify X-Artifact-SHA256; validate media type, name, and non-empty size.
• Atomically register metadata beside local Artifact bytes without embedding payloads in Run JSON.
• List and download registered artifacts only after tenant-scoped Run authorization, while preserving legacy RunResult references.
• Snapshot the agent-compose command context before Artifact persistence so completed processes are not reclassified as timeouts during post-processing.
• Update OpenAPI, README, runbook, changelog, and project status.

Security boundaries

• Authentication and tenant ownership are checked before request bytes are stored.
• Cross-tenant upload, list, and download remain existence-hiding 404 responses.
• SAS_MAX_BODY_BYTES is enforced with structured 413 artifact_too_large errors.
• Stored paths remain Run-scoped; metadata is published atomically and symlink/non-regular downloads are rejected.
• No arbitrary shell execution, Provider call, customer data, or live Sandbox action was added or performed.

Verification

• make SAS_EXECUTOR=mock SAS_VERIFY_AGENT_COMPOSE=0 verify
• make SAS_EXECUTOR=mock SAS_VERIFY_AGENT_COMPOSE=0 race
• Actual Mock HTTP smoke: create 202, terminal succeeded, upload 201, list count 2, download 200, cross-tenant 404, oversized upload 413.

Non-goals

This local-file baseline does not claim chunked upload, presigned object storage, HA, encryption-at-rest, or lifecycle management. Those belong to the later object-storage phase.

Rollback

Stop new uploads, revert this PR, and retain SAS_ARTIFACT_DIR until the retention decision is complete. Older binaries ignore .metadata, so API-uploaded files remain on disk but are not listed or downloaded.

Closes #15