feat: support S3-backed context stores

[beinan]

Background

Context URIs are currently assumed to be local filesystem paths. Cloud-hosted agents need to persist shared context into remote object storage.

Proposal

• Teach to detect URIs and initialize a Lance dataset backed by S3 via object store configuration.
• Surface optional AWS credentials/configuration through environment variables and/or explicit parameters.
• Update the Python path so S3 URIs are passed straight through and validated early.

Design Notes

• Leverage Lance's existing integration (, ) rather than reimplementing S3 clients.
• Allow per-run temporary credentials by honoring standard AWS env vars and the Lance object store configuration file.
• Keep the URI format aligned with Lance's convention () so downstream tooling interoperates cleanly.

Acceptance Criteria

• A context created with an S3 URI can append, checkout, and search records without regressions.
• Python integration tests cover smoke usage against moto or a stubbed S3 endpoint.
• Documentation explains required AWS permissions and credential discovery order.

[beinan]

implemented in #20