Phase 2: normalize environment mode and message-prefix dev parsing

[cgrpa]

Summary

• implement issue PRD: Remote Secret Loading via Azure Key Vault (Staging First) #22 phase-2 runtime mode cleanup with a shared HostEnvironmentMode contract
• switch development command handling from bot-prefix mutation to message-prefix parsing (test- applied at parse time)
• keep canonical bot identities (gemini, grok) and route dev messages via BotRegistry(messagePrefix)
• harden Terraform secret bootstrap workflow and docs (upsert-required-secrets.sh, Terraform outputs/readme)
• refresh root/app docs for DSharpPlusBot naming + Development environment behavior

Key Changes

• add RuntimeMode + HostEnvironmentMode behavior tests
• refactor DiscordWorker to inject message prefix into BotRegistry
• refactor BotRegistry.TryGetBot to enforce optional message prefix and then parse canonical bot prefix
• remove env-prefix constructors from bot config classes and update related tests

Validation

• dotnet build src/dotnet/TheSexy6BotWorker.Tests/TheSexy6BotWorker.Tests.csproj --no-restore
• DOTNET_ROLL_FORWARD=Major dotnet test src/dotnet/TheSexy6BotWorker.Tests/TheSexy6BotWorker.Tests.csproj --filter "FullyQualifiedName~HostEnvironmentModeTests|FullyQualifiedName~BotRegistryTests|FullyQualifiedName~BotConfigurationIntegrationTests|FullyQualifiedName~GeminiBotConfigurationTests|FullyQualifiedName~GrokBotConfigurationTests|FullyQualifiedName~BotConfigurationExtensionsTests|FullyQualifiedName~MarkdownIntegrationTests"
• full DOTNET_ROLL_FORWARD=Major dotnet test src/dotnet/TheSexy6BotWorker.Tests/TheSexy6BotWorker.Tests.csproj still has existing Perplexity integration failures (401 Unauthorized) pending local key refresh