feat: bash completion works on stock macOS bash 3.2 (no bash-completion dep)

[alexzhangs]

Summary

Tab completion in 0.6.0 silently does nothing on stock macOS because:

• /bin/bash is 3.2.57
• bash-completion@2 requires bash 4+ and bails silently
• completions/xsh.bash's first line is _init_completion || return → returns immediately

Three small coordinated changes make it work out of the box:

1. completions/xsh.bash — _init_completion fallback shim

Defined only if not already present (so a real bash-completion load wins, whether earlier or later). Sets the same four caller-local variables the completer reads. Defensive cword <= 0 guard matches real _init_completion semantics and avoids bash 3.2's bad-array-subscript error on ${COMP_WORDS[-1]}.

2. .xshrc — auto-source the completion file

Users don't have to touch ~/.bash_profile or rely on /etc/bash_completion.d/ discovery (which doesn't exist on stock macOS).

3. install.sh — drop the now-redundant /etc/bash_completion.d/xsh copy

That copy was a no-op on macOS (no such dir) and double-sourced on Linux once .xshrc auto-source landed.

Verified

Stock /bin/bash 3.2.57, no bash-completion package installed:

$ xsh <TAB><TAB>
load unload update list imports unimports import unimport help debug
version versions upgrade calls call exec log lib-manager lib-dev-manager
+ 200+ LPUEs from loaded libs

$ xsh load <TAB><TAB>
xsh-lib/aws  xsh-lib/core  xsh-lib/git

Also verified the shim does NOT override a pre-existing _init_completion (when bash-completion is loaded earlier).

Test plan

• CI green (matrix: bash 3.2 macOS / bash 4.4 rocky / bash 5.x macOS+Linux)
• After merge: existing 0.6.0 users get the fix by re-running bash install.sh -f -s or via xsh upgrade

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.29%. Comparing base (e5df7b1) to head (2bbd27a).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #28      +/-   ##
===========================================
+ Coverage    63.13%   63.29%   +0.15%     
===========================================
  Files            3        3              
  Lines          792      790       -2     
===========================================
  Hits           500      500              
+ Misses         292      290       -2     

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