fix: make bash completion work without the bash-completion package

[gtsiolis]

Fixes DEVX-950 (reported by DevRel).

Problem

On stock macOS (bash 3.2.57, no bash-completion package) every Tab press failed with

_get_comp_words_by_ref: command not found

and produced zero completions. Cobra's generated bash script depends on that function from the bash-completion package on both of its init paths — the "Mac fallback" it embeds assumes Homebrew's legacy bash-completion v1.3 is installed, and a truly stock Mac has no bash-completion at all. (Upstream treats this as a documented dependency, see spf13/cobra#2213; kubectl/gh/docker all answer with "install bash-completion" docs.) Zsh was unaffected because Cobra's zsh script is self-contained.

Fix

• Prepend a guarded pure-bash _get_comp_words_by_ref fallback to the generated script (cmd/completion.go), defined only when the bash-completion package is absent — the approach git ships in git-completion.bash, which is why git completion works on stock Macs. Unlike git's fallback, it uses COMP_LINE to recover word adjacency, matching the real package's reassembly semantics for shapes like --config=x vs --config= x. The body is bash 3.2 compatible.
• Generate both the fallback and Cobra's script body against the writer resolved at execution time (delegating to Cobra's captured RunE would split the two halves across writers when SetOut is called after NewRootCmd).
• Replace the bash help text: the default recommended source <(lstk completion bash), which is a silent no-op on bash 3.2 (sourcing a process substitution reads nothing there); recommend eval "$(lstk completion bash)" instead, which works on every bash.

When the bash-completion package is installed, the guard skips the fallback and behavior is unchanged. Zsh/fish/powershell output is byte-identical to before.

Testing

• 6 integration tests drive the real generated script in a bare bash (--noprofile --norc, minimal env — exactly the stock-macOS condition, reproducible on any CI OS): completion works without the package, whitespace-vs-adjacent wordbreak reassembly parity (4 fixtures), and the fallback yielding to an installed package. All failed before the fix with the exact reported error.
• 2 unit tests pin the single-writer wiring and --no-descriptions passthrough.
• Verified end-to-end on stock /bin/bash 3.2.57 and Homebrew bash 5.2 in a clean env: eval "$(lstk completion bash)" + Tab completes lstk st → start status stop, including --flag=value and pod: word shapes.