zsh: only export SSH_AUTH_SOCK when the socket exists

[presto8]

The symlink to the agent socket is created conditionally, but the
export ran unconditionally. With setup_ssh disabled -- or simply with
no agent running -- every shell got SSH_AUTH_SOCK pointing at a
non-existent ~/.ssh/ssh_auth_sock, which breaks agent forwarding that
would otherwise have worked.

Guard the export with -S (which follows the symlink, so a dangling
link is correctly rejected), and mkdir ~/.ssh before linking into it
so the ln does not error on hosts where setup_ssh never ran.

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🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The change is small, locally contained to shell init logic, and corrects a concrete failure mode without introducing new behavioral risk.

Pull request overview

This PR fixes an SSH agent environment propagation issue in prestobuntu.zsh by ensuring SSH_AUTH_SOCK is only pointed at the stable ~/.ssh/ssh_auth_sock path when that path actually resolves to a live socket, avoiding broken agent forwarding in shells where no agent is running.

Changes:

• Create ~/.ssh before attempting to symlink the agent socket into it.
• Guard the SSH_AUTH_SOCK export behind a [[ -S ... ]] check so a dangling symlink is not exported.
• Add inline commentary explaining the rationale and behavior (including symlink-following semantics).

File summaries

File	Description
prestobuntu.zsh	Prevents exporting a non-existent SSH_AUTH_SOCK by exporting only when the stable symlink resolves to a live socket; also ensures ~/.ssh exists before linking.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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