feat(session): sk session digest + stats — per-session deep-dive and aggregate stats

[magicpro97]

User Story

As a developer, I want sk session digest <id> for a per-session summary and sk session stats for aggregate stats by label/branch.

Acceptance Criteria

• sk session digest <prefix>: metadata, turns summary, knowledge entries, files touched, cost
• sk session stats [--by label|branch|day|week] [--since DATE] [--limit N]
• Both support --json
• Tests in test_fixes.py (20+)

Note: Combines issue #719 (digest) with stats subcommand.

Priority: HIGH