fix(config): normalize keys in GetYamlConfig to match SetYamlConfig

[GraemeF]

Summary

GetYamlConfig was not normalizing key aliases (e.g., sync.branch → sync-branch), causing bd config get sync.branch to return "not set" even when the value was correctly stored.

SetYamlConfig already normalized keys, but GetYamlConfig did not, leading to a confusing mismatch where set appeared to work but get could not find the value.

Reproduction

bd config set sync.branch beads-sync
# Output: Set sync.branch = beads-sync (in config.yaml)

bd config get sync.branch  
# Output: sync.branch (not set in config.yaml)

cat .beads/config.yaml | grep sync
# Output: sync-branch: "beads-sync"  # It IS there!

Fix

Added key normalization to GetYamlConfig to match SetYamlConfig:

func GetYamlConfig(key string) string {
    if v == nil {
        return ""
    }
    normalizedKey := normalizeYamlKey(key)  // Added this line
    return v.GetString(normalizedKey)
}

Test Plan

• Added TestGetYamlConfig_KeyNormalization - verifies aliased key lookup works
• Existing TestNormalizeYamlKey still passes
• Existing TestSetYamlConfig_KeyNormalization still passes
• All ./internal/config/... tests pass

Fixes #873