Allow features.*.enabled flag syntax

[pakrym-oai]

Why

Simple feature flags only accepted boolean values under [features], while feature flags with extra configuration accepted an enabled field. That made features.<feature>.enabled = true work for only a couple of special-cased feature configs instead of uniformly across the feature table.

What changed

• Added a shared FeatureToggleToml shape so simple feature flags accept either true/false or { enabled = true/false }.
• Updated config schema generation and the generated schema fixture so all simple feature keys advertise both shapes.
• Updated under-development feature warnings and config tests to cover table-style enables in root and profile feature tables.

Validation

• cargo test -p codex-features
• cargo test -p codex-core simple_features_can_be_configured_with_enabled_tables
• cargo test -p codex-core config_schema_matches_fixture
• cargo test -p codex-config

Documentation

The public config docs should mention the new features.<feature>.enabled = true form alongside the existing boolean form.