Reorganize schema files into schemas/ directory with shared refs and space-level config

[mindsocket]

Foundational restructuring to support multiple schemas and composability.

Tasks

• Create schemas/ directory
• Move schema.json → schemas/general.json; update all code references to the default schema path
• Extract common $defs into schemas/_shared.json (status, priority, wikilink, assessment, base entity properties) to eliminate repetition across schema files
• Rewrite schemas/general.json to $ref the shared defs
• Promote schema key in config.json to space level — each space can specify its own schema file; top-level default remains schemas/general.json
• Ensure --schema CLI option continues to work correctly with the new layout

Why

This is the prerequisite for adding a strict_ost schema and future composable schemas. Separating shared definitions avoids duplication and drift between schemas.

Relates to #3 (research findings).

[mindsocket]

Implemented in 3c79468. Schema files reorganised into schemas/, shared defs extracted to _shared.json, space-level schema config added, and AJV registry loading in place for composable future schemas.