[FEATURE]: YAML config parsing for RUST Core with routing rules.

[terylt]

Epic: CPEX Rust Core (#12)

Summary

Implement the unified YAML config parser in cpex-core (config.rs). Parses the single config format that combines global policies, defaults, plugin declarations, and per-entity routes into typed Rust structs via serde.

Motivation

Currently, plugins must be registered programmatically. Config-driven registration enables declarative plugin setup from a single YAML file. We will add routing rules to the current plugin configuration, and provide experimental integrations with APL eventually.

Scope

• CpexConfig top-level struct: global, plugins, routes
• GlobalConfig: policies (named policy groups with all reserved), defaults (per entity type), delegation, identity, session
• PluginDeclConfig: per-plugin declaration (name, kind, hooks, mode, priority, capabilities, conditions, config)
• RouteEntry: entity matcher (tool/resource/prompt/llm), meta (tags, scope, properties), policy, post_policy, args, result, taint
• Validation at load time: duplicate plugin names, unknown hook names, invalid mode/capability values
• Integration with PluginManager — config drives register_handler calls

Acceptance Criteria

• Parse the example YAML from the unified config proposal without errors
• Type validation catches malformed configs at startup
• PluginManager can be initialized from a config file path
• Route matching: exact name > name list > glob > meta tag match > when expression > defaults

References

• Plugin Framework Spec
• unified config proposal
• APL DSL spec
• Current stub: crates/cpex-core/src/config.rs