Skip auto-save during boot grace to prevent restore clobber

[vivek8031]

Problem

When @continuum-restore on is set, a status-bar driven continuum_save.sh can fire after the bootstrap (single-session) tmux state has been created but before the backgrounded continuum_restore.sh has finished restoring the saved sessions. The save then overwrites the resurrect last symlink with the bootstrap-only state, so auto-restore reads a near-empty file on the next boot and brings nothing back.

This matches symptoms reported in:

• Auto restore via continuum-restore often failing #90 (Auto restore via continuum-restore often failing — ~50% failure rate)
• Current and last active sessions are not correctly saved and restored #94 (saved files contain incomplete session data)
• joeywrites.dev "How to fix tmux resurrect when it fails to load your previous sessions" — manual ln -sf workaround for exactly this clobbered-symlink state

Reproduction

# With @continuum-restore on and any non-empty saved state:
tmux kill-server
tmux new-session -d -s bootstrap   # status-bar fires before restore completes
# `last` symlink now points at a single-session save; restore reads it -> nothing useful

Fix

Add an in_boot_grace_window guard to continuum_save.sh's main loop. The guard only activates when @continuum-restore on is set (so users who don't auto-restore aren't affected) and blocks auto-save until uptime exceeds @continuum-restore-max-delay + 30s. The 30s buffer covers the actual restore work after the delay window for restore-start expires.

Verification

Tested against a separate tmux socket (-L test-patch) with @resurrect-save-script-path pointed at a probe script that logs invocations:

Test	Uptime	@continuum-restore	Expected	Got
1	21 s	on	skip	skip ✅
2	42 s	on	save	save ✅
3	1 s	off	save	save ✅

Compatibility

• No new tmux options introduced — reuses existing @continuum-restore-max-delay (default 10).
• Behaviour unchanged when @continuum-restore is off or unset (default).
• Behaviour unchanged after the boot window expires.