Session index survives git reset — move out of gitignored path

[fazxes]

Problem

docs/sessions/index.md is in .gitignore. Every git reset --hard origin/main wipes it. Historical session data is lost between daemon restarts.

Fix

Either:
A. Remove from .gitignore and commit during housekeeping (like sync_github_tasks does)
B. Move to ~/.nightshift/session-index.md outside the repo

Option A is simpler and keeps everything in one place.

Acceptance Criteria

• Session index persists across daemon cycles
• Historical entries not lost on git reset
• Daemon housekeeping commits index updates

[fazxes]

Converted to task #130