feat: context depth indicator per session

[matoushavlena]

Summary

Priority: P0 | Category: Attention Routing

Show how invested the user is in each session — so deep sessions feel instinctively more important than shallow ones at a glance.

Problem

When scanning your session list, there's no signal for which sessions you've invested the most in. You might abandon a session where you've made 15 decisions and spent 30 minutes for one that just started. That's expensive: re-entering a deep session later costs far more than continuing it now.

Proposed Solution

A visible indicator of context depth per session, based on:

• Number of turns
• Number of approvals given
• Time spent in-session
• Number of decisions made

This could manifest as:

• Depth meter/bar that fills with investment
• Simple label — "deep", "medium", "shallow" based on thresholds
• Visual weight — thicker border, brighter glow that increases with investment

The indicator should make you instinctively protective of deep sessions. When two sessions both need you, the deeper one should feel more important at a glance.

Acceptance Criteria

• Each session shows a visual representation of how invested the user is
• The indicator updates as the user interacts with sessions
• "Deep" sessions are visually distinct from "shallow" ones without requiring reading text
• Integrates with priority scoring as an input signal

Open Questions

• What's the right metaphor? Depth bar? Heat? Weight?
• Should this decay over time? (If you haven't touched a session in hours, does your "depth" fade?)
• How does this interact with token burn visualization (feat: token burn visualization per session #18)? Separate indicator or combined?