fix: add attach error status mode

[hyang214]

Summary

• add --error-status-mode shell|attach to shpool attach
• preserve the current shell-status behavior by default
• let automation opt into a non-zero status when attach fails, including busy-session and invalid-name failures
• return success after a successful attach in attach mode even when the attached shell exits non-zero

Root cause

shpool attach currently returns success when a session is busy, so callers cannot distinguish a successful attach from the already has a terminal attached path. The existing shell-status behavior also means the eventual shell exit code answers a different question from whether attach itself succeeded.

The new mode keeps the existing behavior compatible while giving scripts and wrappers an explicit attach-status contract.

Closes #153.

Validation

• cargo +1.85.0 check --workspace
• cargo +1.85.0 test --all-features --test attach (55 passed, 9 platform-specific tests ignored)
• cargo +nightly fmt -- --check
• cargo +nightly clippy --all-targets --all-features -- -D warnings -A mismatched-lifetime-syntaxes (allows the existing warning in test_hooks.rs)

AI usage

AI assistance was used to inspect the codebase, draft the implementation and tests, and prepare this PR description. I reviewed the resulting diff and ran the validation commands above locally.

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[ethanpailes]

This looks mergable as is. Thanks!

Let's see if CI is happy.

[ethanpailes]

Looks like we'll need a CLA sig before we can merge.

[ethanpailes]

Oh, one nit: this is a feature not a bugfix.