feat(slackbotv2): configure model and reasoning aliases

[decofe]

Summary

• add chart-configurable model and reasoning aliases
• apply the same aliases to flags, channel defaults, and natural-language selection
• validate configuration and cover custom aliases in Slackbot tests

Testing

• pnpm --filter slackbotv2 test (253 passed, 1 skipped)
• bun test services/slackbotv2/test/overrides.test.ts (52 passed)
• jq empty contrib/chart/values.schema.json
• git diff --check
• pnpm --filter slackbotv2 run check:types (blocked by three pre-existing fetch.preconnect errors in test/slack-user.test.ts)
• Helm render not run because helm is unavailable in this sandbox

Prompted by: @Zygimantass