Add unit tests for limits.AdaptiveDepth

[JordanCoin]

Problem

limits/limits.go has no test file. AdaptiveDepth is a hub file (imported by 3 packages) and its threshold logic determines tree depth for all repos.

Suggested fix

Add a table-driven test in limits/limits_test.go covering:

• Unknown file count (<=0) → depth 2
• Small repo (e.g. 100 files) → depth 4
• Medium repo (e.g. 3000 files) → depth 3
• Large repo (e.g. 6000 files) → depth 2
• Boundary values at 2000 and 5000

Quick win for confidence in a high-impact function.