chore: remove .claude/skills/ maintenance skills (moved to deepgram/agents) [no-ci]

prepare-regen and review-regen are SDK-team workflow skills targeted
at us, not at SDK users. They've been generalized and consolidated
into the internal deepgram/agents repo (PR #16) where the SDK team
installs them once and gets them across every SDK they touch.

This SDK's user-facing skills (under .agents/skills/ if present) are
unchanged -- those are for SDK consumers and stay where they are.

See: deepgram/agents#16

Author: lukeocodes