[RUC-INT] Provide Session Export API for entrenar

[noahgift]

Context

The entrenar Experiment Tracking Spec v1.8.0 references ruchy REPL session integration (§14.1).

Epic Reference

• Epic: ENT-EPIC-001
• Related: entrenar#64 (ENT-033)

Integration Requirements

Session Export API

entrenar needs to import ruchy sessions as experiment artifacts:

• Session history as literate document
• Code cells + outputs preserved
• Timestamps for experiment lineage

Expected API

pub struct RuchySession {
    pub cells: Vec<Cell>,
    pub started_at: DateTime<Utc>,
    pub ended_at: Option<DateTime<Utc>>,
}

impl RuchySession {
    pub fn to_literate_document(&self) -> LiterateDocument;
    pub fn to_experiment_artifact(&self) -> Artifact;
}

Acceptance Criteria

• Public session export API
• LiterateDocument conversion
• Artifact serialization

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