chore: remove maintenance skills from .claude/ and .agents/ (moved to deepgram/agents) (#702)

## Summary

Removes maintenance-flavored skills targeted at the SDK team, not at SDK
users:

- `.claude/skills/prepare-regen.md`
- `.claude/skills/review-regen.md`
- `.agents/skills/deepgram-python-maintaining-sdk/` (duplicated
AGENTS.md content; its own footer admitted so)

These have been generalized and consolidated into the internal
`deepgram/agents` repo so the SDK team installs them once and gets them
across every Deepgram SDK we maintain (Python, JS, Java, Kotlin, Go,
Rust, .NET, Swift).

- Companion PR: https://github.com/deepgram/agents/pull/16

## What's not changing

- `AGENTS.md` in this repo is unchanged. It remains the SDK-specific
source of truth for the frozen-file list, validation command, and any
language-specific maintainer notes.
- The user-facing skills under `.agents/skills/`
(`deepgram-python-speech-to-text`, `deepgram-python-voice-agent`, etc.)
are untouched. Those are for SDK consumers and stay where they are.
- The Fern regen workflow itself is unchanged. Just the workflow
guidance moves to the internal install.

## After this lands

The canonical maintenance reference is now `deepgram/agents` (workflow)
+ this repo's `AGENTS.md` (specifics). No information is lost —
`AGENTS.md` has the full per-repo detail.

SDK-team members install the new skills via `deepgram/agents` and run
the regen workflow exactly as before. Triggers like "fern regen" /
"freeze classification" / "prepare regen" / "review regen" continue to
work; they're now driven by the generalized skills in `deepgram/agents`.