refactor: config early workspace discovery

[gvozdvmozgu]

Summary

Stacked PR 3/10: based on config-early-cli-migration; review and merge after that PR.

• Split workspace discovery out of the engine layer.
• Keep discovery reusable without coupling it directly to engine execution.

Testing

• Validated with the full stacked branch:
• cargo +1.96.1 fmt --check
• cargo +1.96.1 check -p sqruff-lib -p sqruff-cli-lib -p sqruff-lsp
• cargo +1.96.1 test -p sqruff --test none_format

[github-actions]

Benchmark for 7b45b4a

Click to view benchmark

Test	Base	PR	%
DepthMap::from_parent	57.8±0.76µs	59.7±3.27µs	+3.29%
fix_complex_query	12.0±0.15ms	12.1±0.07ms	+0.83%
fix_superlong	114.7±5.85ms	116.5±3.54ms	+1.57%
parse_complex_query	4.0±0.05µs	4.1±0.06µs	+2.50%
parse_expression_recursion	6.8±0.04µs	7.1±0.14µs	+4.41%
parse_simple_query	1043.4±23.79ns	1047.1±16.59ns	+0.35%

[github-actions]

Benchmark for 859cc09

Click to view benchmark

Test	Base	PR	%
DepthMap::from_parent	59.4±0.87µs	59.5±0.76µs	+0.17%
fix_complex_query	12.3±0.27ms	12.2±0.08ms	-0.81%
fix_superlong	130.7±7.34ms	129.8±9.78ms	-0.69%
parse_complex_query	4.9±0.07µs	4.1±0.03µs	-16.33%
parse_expression_recursion	8.2±0.26µs	7.2±0.77µs	-12.20%
parse_simple_query	1082.2±25.15ns	1040.9±13.99ns	-3.82%