Add FableBot: standalone Discord bot-account poster project

[ernop]

Summary

New FableBot/ console project in MultiImageClient.sln: posts text and/or file attachments into one Discord channel as a bot account (Discord REST v10, Authorization: Bot <token>, no gateway in v1). The v1 target is a private channel on the owner's server. Full requirements, settled decisions, and the owner setup checklist are in docs/fablebot-discord-prd.md.

Workflow impact

• No existing run mode changes. The vibecoders webhook sender is untouched.
• Settings.Validate() gains fail-closed validation for the new pair FableBotDiscordBotToken + FableBotDiscordChannelId (both blank = disabled; one without the other, malformed token, or non-numeric channel id fails settings load).

Required settings updates

Optional. To enable FableBot, add to settings.json:

"FableBotDiscordBotToken": "<raw bot token>",
"FableBotDiscordChannelId": "<numeric channel id>"

Works with a newly created bot application or the existing SocialAI bot token (checklist for both in the PRD).

Usage

dotnet run --project FableBot -- --check
dotnet run --project FableBot -- --message "hello" --file saves/example.png

Behavior

• Identity + channel are resolved before every post; a bad token or invisible channel is a specific error, and the channel lookup supplies the guild id for the printed jump link.
• Local fail-closed limits: 2000 chars, 10 files, 10 MiB/file, no empty messages — rejected, never truncated or split.
• allowed_mentions: {"parse": []} on every post, so pasted mentions never ping.
• Attachment content types from magic bytes (PNG/JPEG/WEBP/GIF/MP4); bytes travel verbatim.
• No retry on 429 in v1; Discord's body (with retry_after) is surfaced as the failure.

Testing

• dotnet build MultiImageClient.sln — 0 errors (warnings pre-existing).
• dotnet test — 247 passed (6 new in FableBotTests: snowflake/token normalization, message limits, content-type sniffing, jump-link identity, settings pair validation).
• CLI smoke: --help, missing settings file (exit 1), settings without FableBot keys (exit 1 with setup pointer).
• A live post needs the owner's bot token, which is not on this VM: after filling in the two settings keys, run dotnet run --project FableBot -- --check, then a real --message post.