refactor: consolidate duplicated helpers into internal/util

[devin-ai-integration]

Summary

Several tiny helper functions were copy-pasted verbatim across packages. This adds a single generic internal/util package and replaces all the duplicates with it, removing the local copies.

New package internal/util:

func Ptr[T any](v T) *T                       // was boolPtr in 2 packages
func Deref[T any](p *T, fallback T) T          // was boolOrDefault
func OrDefault[T comparable](value, fallback T) T // was valueOrDefault (x3) + intOrDefault
func FirstNonEmpty(vals ...string) string      // was firstNonEmpty (x2) + firstNonEmptyString

Call sites updated:

• boolPtr → util.Ptr in config/convert.go, service/store/sqlite_store.go
• valueOrDefault/intOrDefault → util.OrDefault, boolOrDefault → util.Deref in config/config.go + config/config_loader.go, and valueOrDefault in extension/codex/catalog.go
• firstNonEmpty/firstNonEmptyString → util.FirstNonEmpty in protocol/openai/adapter.go, extension/deepseek_v4/{deepseek_v4,state}.go, service/server/adapter_dispatch.go
• Removed a dead valueOrDefault (no callers) in service/provider/manager.go

util has no imports, so no import cycles are introduced. Behavior is identical — OrDefault uses the zero value (""/0) as the "empty" sentinel, matching the previous per-type helpers.

Testing

• go build ./..., go vet ./... clean
• go test ./... passes (all packages), including new internal/util unit tests

Link to Devin session: https://app.devin.ai/sessions/2d5f007ca93b447f9171f0bb1fbf757a
Requested by: @ZhiYi-R

[devin-ai-integration]

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