docs: split provider-specific docs out of README and add provider-dev guide

[chr1syy]

Summary

The README had grown to ~250 lines mixing kernel-level info with three big chunks of Discord/voice/whisper-specific detail. New file structure:

New file	Contents
`docs/discord.md`	Bot setup, OAuth invite URL + permissions integer, full `DISCORD_*` env table, slash commands, runtime behavior, security, troubleshooting
`docs/voice.md`	whisper.cpp setup (production + source flows), behavior notes, config table incl. the previously-undocumented `WHISPER_LANGUAGE`
`AGENTS-providers.md` (+ `CLAUDE-providers.md` symlink)	Deep-dive provider-development guide: `BridgeProvider` contract, `IncomingMessage` / `OutgoingMessage` / `KernelContext` shapes verified against `src/core/types.ts`, file-layout convention, DB/env conventions, voice-transcription integration pattern, shipping checklist

Refactored existing files:

• `README.md` — replaced inline Discord/voice/permissions sections with a Providers table and links. Kept kernel-agnostic content (install, build, tests, migration, data storage).
• `AGENTS.md` — slimmed the Discord-provider file list to a one-liner pointing at `docs/discord.md`, replaced the 4-bullet "Adding a new provider" with a TL;DR + link to `AGENTS-providers.md`.
• `docs/architecture.md` — replaced its duplicate "Adding a new provider" section with a link to `AGENTS-providers.md` (single source of truth).
• `.env.example` — added `WHISPER_LANGUAGE` for parity with `voice.md` (was already read by `src/core/config.ts`, just wasn't documented).

No code changes; no version bump.

Test plan

• All internal markdown links resolve (manually verified all `*.md` link targets exist)
• Type signatures in `AGENTS-providers.md` cross-checked against `src/core/types.ts` (`IncomingMessage`, `OutgoingMessage`, `IncomingAttachment`, `KernelContext`, `EnqueueOptions`)
• Voice-transcription flow in `AGENTS-providers.md` cross-checked against `src/providers/discord/messageCreate.ts` (`contentOverride` + `attachmentsOverride` pattern)
• CI: lint+test+CodeRabbit on this PR

🤖 Generated with Claude Code

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Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 60e03260-5888-46d4-9168-a559d80d5419

📥 Commits

Reviewing files that changed from the base of the PR and between 747976c and e1b8a0a.

📒 Files selected for processing (8)

• .env.example
• AGENTS-providers.md
• AGENTS.md
• CLAUDE-providers.md
• README.md
• docs/architecture.md
• docs/discord.md
• docs/voice.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/split-provider-docs

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