M25.1: TrustLevel enum, SkillSource, and SQLite migration

[bug-ops]

Parent: #418

Scope

Implement the trust model data layer and storage.

Tasks

• Add TrustLevel enum (Trusted, Verified, Quarantined, Blocked) to zeph-skills/src/trust.rs
• Add SkillSource enum (Local, Hub, File) to zeph-skills/src/trust.rs
• Extend SkillMeta with trust_level, source, content_hash fields
• Add skill_trust SQLite table migration in zeph-memory
• Implement CRUD operations for skill trust records
• Compute blake3 hash of SKILL.md content at load time
• Local skills default to Trusted (backward compatible), unknown skills default to Quarantined
• Add [skills.trust] config section to zeph-core config types
• Update config/default.toml with trust defaults
• Unit tests for TrustLevel ordering, transitions, and DB operations

Acceptance Criteria

• Existing skills load as Trusted with no behavioral change
• New skill_trust table created on migration
• SkillMeta carries correct trust level from DB lookup