feat: add attach background mode

[tobwen]

Summary

Adds -b/--background flag to shpool attach for creating sessions with commands and immediate detach. resolves #320

Changes

• Added -b/--background CLI flag (mutually exclusive with --attach-only)
• Refactored attach_impl in libshpool/src/attach.rs to return connected client before pipe_bytes
• Background mode skips interactive signal handling
• Issues explicit detach after closing attach stream
• Version mismatch handling is non-interactive in background mode

Behavior

With background flag:

• Session creation and command initialization proceed normally
• After closing attach stream, explicit detach is issued
• Returns immediately without waiting for command completion
• Error if attempting to attach to busy session with background flag

Rationale

Useful for:

• Starting background jobs in named sessions for later monitoring
• Automating session setup without manual intervention
• Batch operations across multiple sessions

Testing

Run integration tests:

cargo test --test attach background

Tests

• busy_background: Error when attaching to busy session with background flag
• background_custom_cmd: Basic background command execution
• busy_background_force: Validates -f + -b restriction

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[ethanpailes]

Just one little nit. Overall, this looks great. Seems like a good feature and the code and tests all look clean.

[ethanpailes]

Looks like there are some lints that are grumpy at you. Feel free to just suppress the too-many-args one.

[ethanpailes]

Looks like there are still some lint failures. To suppress the failure, you should add a #[clippy::allow(...)] block above the run method that has the lint failure (https://github.com/shell-pool/shpool/actions/runs/22588665489/job/65441921530?pr=321)

[tobwen]

To suppress the failure, you should add a #[clippy::allow(...)] block above the run method that has the lint failure

Sorry for the noise. I added it now.

[ethanpailes]

Looks like a flaky test since they passed before and the only change is a lint suppression. I'm just going to merge.