[chore] replace substring-based isBusyErr with typed sqlite error sentinel

[kunallanjewar]

Summary

Two copies of isBusyErr (one in internal/lore/tx.go, one in internal/quest/accept.go) parse SQLite busy-state via string matching on the error message. modernc.org/sqlite exposes a typed sqlite.Error value with a numeric code, so the check can be replaced with a single errors.As / errors.Is-based helper.

Affected files

• internal/lore/tx.go (line 74)
• internal/quest/accept.go (line 176)

Acceptance

• Single shared helper in a sensible package (e.g. internal/storage/).
• Helper uses typed error sentinels rather than strings.Contains.
• Both call sites (lore tx + quest accept retry loops) use the new helper.
• Existing tests pass; no behavioral change expected.