fix: env blocks not setting variable for SSH script and terminal blocks

[BinaryMuse]

Fixes #376

#319 made it so we only set $ATUIN_OUTPUT_VARS if it's included in the script text; since environment variables are often read by sub-programs, I opted not to go that route here.

[greptile-apps]

Greptile Overview

Greptile Summary

Fixes environment variable propagation to SSH sessions for script and terminal blocks. Previously, env blocks only set variables locally but not for remote SSH execution.

The fix introduces build_env_exports() that generates properly escaped export statements for all environment variables, using POSIX single-quote escaping to handle special characters safely.

• Script blocks: prepends env exports to code sent over SSH
• Terminal blocks: writes env exports to SSH PTY after opening
• Local execution: unchanged, continues using native env mechanisms

Important Files Changed

Filename	Overview
crates/atuin-desktop-runtime/src/ssh/ssh_env.rs	Added new module with shell escaping logic for env vars, includes comprehensive tests
crates/atuin-desktop-runtime/src/blocks/script.rs	Now exports env vars for SSH script execution using new build_env_exports function
crates/atuin-desktop-runtime/src/blocks/terminal.rs	Now writes env exports to SSH PTY before setting ATUIN_OUTPUT_VARS

[greptile-apps]

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