Add experimental Rust SDK API

[Fletcher-Alderton]

Summary

• add an experimental things3_cloud::sdk module with typed read and mutation APIs for app integrations
• reuse existing command planners for mutation payload generation so CLI behavior stays unchanged
• extract shared create planners for projects, areas, and tags so CLI and SDK use one source of truth for those creates
• expose resolved task JSON fields for Rust SDK consumers and document SDK usage in the README

Notes

• The CLI remains the stable user interface; this adds an optional Rust API surface for consumers that should not shell out to things3.
• Mutation methods commit the same WireObject changes produced by command planners.
• The SDK supports live cloud reads/writes, cache-only reads, journal fixtures, and dry-run writes for tests.

Validation

• cargo fmt --check (passes; stable rustfmt warns that nightly-only rustfmt.toml options are ignored)
• cargo clippy --all-targets --all-features (passes; existing large_enum_variant warning remains in src/wire/wire_object.rs)
• cargo test new_payload
• cargo test sdk::tests
• cargo test cli_trycmd_projects
• cargo test cli_trycmd_areas
• cargo test cli_trycmd_tags
• cargo test --all-targets

No CLI behavior changes intended.