Update tests, fixtures, and docs for `.dual/` model

[jeevanpillay]

Summary

Update all test fixtures, integration tests, and documentation to reflect the .dual/settings.json model.

Tests

tests/fixtures/mod.rs

• create_fixture_hints() writes .dual/settings.json (JSON) + .devcontainer/devcontainer.json
• Remove TOML fixture writing

src/config.rs tests

• All parse_dual_config / write_dual_config tests use JSON
• load_hints tests create .dual/settings.json
• Remove TOML-specific parse tests
• Add: missing settings.json error test
• Add: required devcontainer field validation

Integration tests

• tests/e2e.rs — verify .dual/ dir created
• tests/fixture_smoke.rs — update for new fixture format

Documentation

README.md

• Update config section: .dual/settings.json schema and examples
• Update quick start: dual add → dual init
• Update CLI command table
• Remove TOML examples, add JSON examples

CLAUDE.md

• Update project structure section
• Update development commands if needed

Verification

• cargo test — all pass
• cargo clippy — clean
• cargo fmt --check — clean
• No references to .dual.toml remain (except migration code)

[linear]

DUAL-68