Rename DeepPrestige to reflect per-character Deep session state

[stphung]

Context

The DeepPrestige struct was designed with the idea that Deep progression would have its own prestige-like reset mechanic. In practice, Deep state simply persists across character prestiges — there is no separate "Deep prestige level" or Deep-specific reset cycle.

The naming DeepPrestige is misleading. It's really just the per-character Deep session state (roster, missions, marks) that happens to survive prestige resets.

Proposed Changes

1. Rename DeepPrestige → DeepSession (or similar) to clarify its role as session state that persists across prestiges, not a prestige system of its own
2. Update field name DeepState.prestige → DeepState.session
3. Update all references across the codebase (types, missions, mercenaries, economy, events, layers, persistence, tick stages, UI, tests, docs)
4. Update documentation in src/deep/CLAUDE.md, CLAUDE.md, and docs/secondary-systems.md

Motivation

• Reduces confusion when reading code ("prestige" already means character prestige)
• Accurately describes what the struct does (holds session state)
• We don't plan to implement a Deep-specific prestige/reset system

Risk

Pure rename refactor — no behavior changes. Migration concern: the JSON field name in ~/.quest/deep.json would change, so a serde alias (#[serde(alias = "prestige")]) should be added for backward compatibility.