feat: optional per-kind schema validation

[DarlingtonDeveloper]

Summary

• Per-kind metadata schema definitions in cortex.toml for write-time validation
• Backward compatible — kinds without schemas pass freely
• Supports required fields, type checking, numeric ranges, and allowed values (enum-like)

Config example

[schemas.decision]
required_fields = ["rationale"]

[schemas.decision.fields.rationale]
type = "string"

[schemas.decision.fields.priority]
type = "number"
min = 1
max = 5

Changes

• crates/cortex-core/src/gate/schema.rs (NEW) — SchemaValidator, field types, violations
• crates/cortex-core/src/gate/mod.rs — Schema variant in GateCheck, check_schema() method
• crates/cortex-core/src/lib.rs — re-exports
• crates/cortex-server/src/config.rs — schemas config parsing
• crates/cortex-server/src/cli/init.rs — schemas field in init config

Test plan

• 10 unit tests (no-schema passthrough, required fields, type mismatch, ranges, allowed values)
• cargo test --workspace passes
• cargo clippy --workspace clean

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