feat(session): sk session digest <id> — per-session deep-dive view

[magicpro97]

User Story

As a developer, I want sk session digest <id> to show what happened in a session: files changed, knowledge added, checkpoints, cost.

Problem

No per-session summary exists. sk retro is aggregate only.

Acceptance Criteria

• sk session digest <prefix>: prefix-matches session id
• Output: header (duration, cost_usd_est, label), files touched, knowledge entries added, checkpoint titles
• --json flag for machine-readable output
• Tests in test_fixes.py

Priority: HIGH