feat: add z -- to jump to last rested directory

[douglasjsellers]

Summary

• Adds z -- command that jumps back to the last directory the user stayed in for at least 5 seconds
• Threshold is configurable via _ZO_REST_THRESHOLD environment variable (default: 5 seconds)
• Implemented entirely in shell templates — no Rust binary changes
• Supported across all 9 shells: bash, zsh, fish, posix, powershell, elvish, nushell, tcsh, xonsh

How it works

Each shell hook now tracks how long the user stays in a directory. When the user changes directories, if they were in the previous directory for >= _ZO_REST_THRESHOLD seconds, that directory is saved as the "last rested" directory. Running z -- jumps to it.

State is session-local (shell variables), similar to how z - / $OLDPWD works.

Time sources (chosen to avoid unnecessary forking)

Shell	Time Source
Bash	$SECONDS
Zsh	$SECONDS
Fish	date +%s
POSIX	date +%s
PowerShell	[DateTimeOffset]::Now.ToUnixTimeSeconds()
Elvish	(time:now)[unix]
Nushell	date now | format date '%s' | into int
Tcsh	`date +%s`
Xonsh	time.time()

Test plan

• Verify cargo test passes (template syntax validation)
• Manual test: cd /tmp && sleep 6 && cd /var && z -- jumps to /tmp
• Manual test: cd /tmp && sleep 1 && cd /var && z -- does NOT update last rested
• Manual test: z -- with no prior rest prints error
• Manual test: _ZO_REST_THRESHOLD=2 lowers the threshold

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