Fix Discord bot token configuration

[shanselman]

Fixes #958

Summary

• write the Discord inline credential to channels.discord.token using a Discord Bot Token field
• replace webhook setup copy with Discord bot/Developer Portal guidance across all five locales
• keep Google Chat webhook configuration unchanged and add focused regressions for the schema path and localized guidance

Validation

• .\build.ps1 — passed; Shared, Cli, WinNodeCli, SetupEngine, and WinUI built successfully
• dotnet test .\tests\OpenClaw.Shared.Tests\OpenClaw.Shared.Tests.csproj --no-restore — passed: 2,724; skipped: 31; failed: 0
• dotnet test .\tests\OpenClaw.Tray.Tests\OpenClaw.Tray.Tests.csproj --no-restore — passed: 1,665; skipped: 0; failed: 0
• focused ChannelsPageDiscordConfigContractTests — passed: 3
• focused ChannelConfigPatchBuilderTests — passed: 22

Real behavior proof

• Launched the current branch build with .\run-app-local.ps1 -NoBuild -Isolated -AllowNonMain; the isolated app started successfully from this worktree.
• UI screenshot: Not verified / blocked. Computer Use approval for the isolated OpenClaw window was declined, so the Discord form could not be navigated or captured without foregrounding the app. No screenshot is claimed.
• Regression proof directly checks that the Discord form emits channels.discord.token, rejects any return of channels.discord.webhookUrl, displays the Discord Bot Token label/placeholder/help, removes webhook guidance from every locale, and preserves channels.googlechat.webhookUrl.
• Rubber-duck review: $env:PYTHONUTF8='1'; python .agents\skills\autoreview\scripts\autoreview --mode local — clean; no accepted/actionable findings.