fix: exclude bash backend from hex package

[mikehostetler]

Summary

• Keep the Bash backend source unguarded in source/Git builds, avoiding guarded Bash.Interop compatibility code.
• Keep the pinned upstream tv-labs/bash Git dependency for dev/test so the backend can compile against Bash.Session.signal/3 before the next Bash Hex release.
• Exclude lib/jido_shell/backend/bash.ex and lib/jido_shell/backend/bash/* from the Hex package until upstream bash publishes the required API.
• Document the temporary source/Git-only Bash backend availability while waiting for the upstream release.

Closes #48.

Validation

• mix compile --warnings-as-errors
• mix test (622 passed, 3 excluded)
• mix hex.build --unpack -o /tmp/jido_shell_hex and confirmed Bash backend files are excluded
• MIX_ENV=prod mix compile --warnings-as-errors from the unpacked package
• mix hex.build