fix: normalize startup session ordering across mixed timestamp formats

[eba8]

Summary

• normalize startup session ordering across mixed timestamp formats
• sort restored sessions by parsed recency in Rust instead of SQLite integer casts
• add a regression test for mixed RFC3339 and unix-with-Z timestamp values

Root Cause

load_sessions_for_startup() was ordering sessions with SQLite expressions like CAST(REPLACE(timestamp, 'Z', '') AS INTEGER).

That works for unix-style values such as 1776438552Z, but breaks for RFC3339 values such as 2026-05-01T14:10:34Z, which collapse to 2026 when cast to an integer in SQLite.

That lets older unix-style rows sort ahead of newer ISO timestamp rows, which can make recent sessions look stale in the library.

Fix

• parse unix-with-Z timestamps when available
• fall back to RFC3339 parsing
• sort restored sessions by parsed last_progress_at, then parsed last_activity_at, then started_at

Validation

• added a regression test covering a newer RFC3339 row and an older unix-with-Z row
• local validation is still pending in this clean worktree because cargo test had to start by downloading the pinned Rust toolchain for origin/main

Closes #262