ci: add shellcheck workflow

[reubeno]

• Adds shellcheck run to CI workflows.
• Fixes existing shellcheck issues in scripts.

[github-actions]

Test Results

    5 files     49 suites   22m 12s ⏱️
2 949 tests 2 949 ✅ 0 💤 0 ❌
9 145 runs  9 145 ✅ 0 💤 0 ❌

Results for commit 663c120.

♻️ This comment has been updated with latest results.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	17.79 μs	17.50 μs	-0.29 μs	⚪ Unchanged
eval_arithmetic	0.15 μs	0.15 μs	0.00 μs	⚪ Unchanged
expand_one_string	1.82 μs	1.86 μs	0.04 μs	⚪ Unchanged
for_loop	31.60 μs	31.91 μs	0.32 μs	⚪ Unchanged
full_peg_complex	59.25 μs	58.81 μs	-0.44 μs	⚪ Unchanged
full_peg_for_loop	6.28 μs	6.29 μs	0.02 μs	⚪ Unchanged
full_peg_nested_expansions	16.33 μs	16.29 μs	-0.04 μs	⚪ Unchanged
full_peg_pipeline	4.38 μs	4.38 μs	-0.00 μs	⚪ Unchanged
full_peg_simple	1.88 μs	1.85 μs	-0.03 μs	🟢 -1.39%
function_call	3.86 μs	3.96 μs	0.10 μs	⚪ Unchanged
instantiate_shell	55.27 μs	55.77 μs	0.49 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	27712.11 μs	27162.45 μs	-549.66 μs	⚪ Unchanged
parse_peg_bash_completion	2108.75 μs	2132.27 μs	23.52 μs	🟠 +1.12%
parse_peg_complex	19.86 μs	19.88 μs	0.02 μs	⚪ Unchanged
parse_peg_for_loop	2.05 μs	2.06 μs	0.01 μs	⚪ Unchanged
parse_peg_pipeline	2.25 μs	2.19 μs	-0.06 μs	⚪ Unchanged
parse_peg_simple	1.17 μs	1.16 μs	-0.00 μs	⚪ Unchanged
run_echo_builtin_command	17.04 μs	17.11 μs	0.07 μs	⚪ Unchanged
tokenize_sample_script	3.43 μs	3.38 μs	-0.06 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
Overall Coverage	🟢 76.31%	🟢 76.31%	⚪ 0%

Minimum allowed coverage is 70%, this run produced 76.31%
Maximum allowed coverage difference is -5%, this run produced 0%

Test Summary: bash-completion test suite

Outcome	Count	Percentage
✅ Pass	1588	75.30
❗️ Error	17	0.81
❌ Fail	150	7.11
⏩ Skip	339	16.07
❎ Expected Fail	13	0.62
✔️ Unexpected Pass	2	0.09
📊 Total	2109	100.00