chore: remove .agents/skills/deepgram-rust-maintaining-sdk (moved to deepgram/agents) (#155)

## Summary

Removes `.agents/skills/deepgram-rust-maintaining-sdk/`. This skill
duplicated `AGENTS.md` content (its own footer says so) and was
internal-facing — targeted at the SDK team running regen cycles, not at
SDK users.

The maintenance workflow has been generalized and consolidated into the
internal `deepgram/agents` repo so the SDK team installs it once and
gets it across every Deepgram SDK we maintain.

- Companion PR: deepgram/agents#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-rust-speech-to-text`, `deepgram-rust-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)
+ each SDK's `AGENTS.md` (specifics). No content 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`.

Author: lukeocodes