Replace settings and tool metadata probing with typed APIs

Parent tracking issue: #4903

Desired Behavior

Settings variants, profiles, hook collections, tool registration, MCP schema aliases, invoke-skill actions, and skill resources should expose explicit typed APIs. Discriminators and class-level metadata should be declared through base classes, class variables, properties, or typed mappings rather than discovered or attached with getattr/setattr.

Scope

• Settings models and profile discriminators
• Tool registry/schema and invoke-skill metadata
• Hook config collection and skill resource assignment
• Excludes lazy package exports and platform compatibility helpers

Acceptance Criteria

• Settings/profile discriminators are declared by their typed base hierarchy.
• Tool registration and MCP schema alias metadata use declared class APIs.
• Hook and skill resource collection uses typed mappings or direct fields.
• Serialization and public schema output remain backward compatible, with targeted regression tests.
• Relevant baseline entries are removed and persisted-settings/API compatibility checks pass.

This issue was created by an AI agent (OpenHands) on behalf of the user.

Start by reading parent issue #4903, then trace the settings/profile models, tool registry and schema metadata, hook config collection, and skill resource assignment named in this issue. Identify the existing probing and baseline entries before changing the APIs. Done means typed declarations replace discovery while serialization, public schemas, persisted-settings checks, and targeted regression tests remain compatible.