fix: create output dir (if not exists) for gen_completions command

[inteist]

this follows standard *nix behavior. i.e. if the folder doesn't exist usually in *nix environments in this case, the folder is created automatically.

• If the directory already exists, it succeeds and does nothing.
• If parent directories are missing, it creates them.
• If the path exists but is a file, it returns an error.
• If permissions prevent creating/accessing it, it returns an error.

Checks

• [ x] I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• [ x] I have checked that there are no existing pull requests for the same thing

[github-actions]

Fossier: Manual Review Requested

@inteist is a new contributor. A maintainer should review this PR before merging.

Score Breakdown

Total Score: 65.0/100 | Confidence: 100% | Outcome: REVIEW

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[greptile-apps]

Greptile Summary

This PR makes completion generation create the requested output directory first. The main changes are:

• Adds fs_err::create_dir_all before writing completion files.
• Keeps the existing generate_to path for shell-specific completion output.
• Leaves stdout generation unchanged when no output directory is provided.

Confidence Score: 4/5

This is close, but the directory mode issue should be fixed before merging.

• The changed path now creates directories while Atuin's restrictive umask is active.

• Shared completion directories can be created as root-only and become unreadable to normal users.

• The stdout path and existing-directory path remain unchanged.

• crates/atuin/src/command/gen_completions.rs

Important Files Changed

Filename	Overview
crates/atuin/src/command/gen_completions.rs	Adds automatic output directory creation before writing generated shell completions.

_{Reviews (1): Last reviewed commit: "fix: create output dir (if not exists) f..." | Re-trigger Greptile}

[greptile-apps]

Creates private completion dirs

AtuinCmd::run sets the process umask to 077 before dispatching commands. With this new create_dir_all, atuin gen-completions --out-dir /usr/local/share/bash-completion/completions can create missing shared completion directories as 0700 when run under sudo, so normal users cannot traverse them and shell completion loading breaks.

[inteist]

Hmmm...

The use case seems a little forced to me.
Most use cases would be to generate completions to STDOUT
We generate completions into a folder to then move it. So IMO this is a non issue.

potential adjustment would be

adding if !matches!(&self, Self::GenCompletions(_)) {

into

impl AtuinCmd {
    pub fn run(self) -> Result<()> {
        #[cfg(not(windows))]
        if !matches!(&self, Self::GenCompletions(_)) {

this needs to be reviewed by someone who better understands various use cases for this.