Move TrustLevel to zeph-core to eliminate reverse dependency

[bug-ops]

Context

zeph-tools depends on zeph-skills solely for TrustLevel enum. This creates a reverse dependency where the tool layer depends on the skill layer.

Solution

Move TrustLevel (and related SkillSource, SkillTrust) to zeph-core::trust or a shared types module. Update all imports.

Acceptance Criteria

• zeph-tools no longer depends on zeph-skills
• TrustLevel accessible from zeph-core
• All crates compile and tests pass

Epic: #832 | Effort: S | Crate: zeph-skills, zeph-tools, zeph-core
Critical path: independent, parallel with AR-007/AR-008/AR-010