Persist logical session tabs across Neovim restarts #519

Context

I run a git-worktree workflow on top of session tabs (one tab per worktree). After closing Neovim and reopening in the same directory, all logical panel tabs are gone — only the currently active session is restored. This forces me to re-open every worktree tab by hand.

Current behavior

• state.session_tabs is rebuilt empty on startup; there is no save/restore of the tab list.
• session_tabs has sync, activate, create, etc., but nothing writes the tab set to disk on VimLeavePre or reads it back on setup.

Desired behavior

On startup, if the current directory matches the directory of the previously open session tabs, restore those tabs (order, active tab, bound directory if #517 lands). If the directory does not match, fall back to current behavior (single session).

Why this is not just my config

The plugin already persists other UI state (window layout via persist_state, model choice, etc.). Session tabs are the natural next step — they are the unit of parallel work, and losing them on every restart makes the multi-tab workflow much less useful.

Happy to test a PR.