refactor: hide MessagePart internal variants from serialization

[bug-ops]

Epic: #1977

MessagePart is #[derive(Serialize, Deserialize)] but contains internal variants:
ThinkingBlock, RedactedThinkingBlock, Compaction — these are Claude-specific artifacts.

Options:

1. Add #[serde(skip)] to internal variants
2. Split into PublicMessagePart and InternalMessagePart

Location: crates/zeph-llm/src/provider.rs

Acceptance Criteria:

• Internal variants do not appear in serialized output
• Persistence (save/load) still works
• Debug dump still captures these blocks

Complexity: Medium | Effort: 2h
Ref: ABS-05