Modernize A-Bot: tooling, CI, bug fixes, tests, and docs

[rramboer]

Summary

Full modernization pass to bring the repo up to current professional standards. Runtime behavior is preserved except where noted.

Tooling & infrastructure

• ESLint 10 flat config with typescript-eslint strict + stylistic type-checked rules; Prettier config, whole repo formatted
• GitHub Actions CI: install (immutable), lint, format check, typecheck, test, build on pushes to master and PRs (Node from .nvmrc, Yarn 4 via corepack)
• Dependabot (weekly npm + actions), .editorconfig, .nvmrc, MIT LICENSE, CONTRIBUTING.md
• Stricter tsconfig (noUncheckedIndexedAccess, exactOptionalPropertyTypes, verbatimModuleSyntax), separate tsconfig.build.json for emit

Bug fixes

• Dev mode (yarn dev via tsx) loaded zero commands — the loader only matched .js; it now accepts .ts/.js
• Race condition: bets were validated at command time but debited at button-settlement time, allowing negative balances across concurrent games — new atomic settleBet transaction re-checks funds
• Reaction roles broke on cache eviction — added message/reaction/user partials and partial fetching
• Slash-command registration via raw REST silently dropped camelCase fields (channelTypes, minValue) — now uses application.commands.set()
• Button handlers hardened against malformed customId JSON

Code quality

• Env-driven config (src/config.ts) — previous hard-coded server IDs kept as defaults so behavior is unchanged without a .env change
• Shared logic extracted to src/lib/ (bet validation, game outcomes, duration formatting, message-link parsing); dead code removed; typed option getters replace stringly-typed args
• Graceful shutdown, unhandledRejection handler, correct error replies for deferred interactions

Tests & docs

• 50 tests (was 13): new test/lib.test.ts covering game-outcome matrices and validation; expanded db tests including the negative-balance race
• README rewritten with accurate command list, env-var table, setup instructions, and CI badge

Deliberate behavior change (flagging for your review)

• The soon.ts easter egg's second branch (crude "CS major" reply) was removed as part of professionalization. The main "Soon:tm:" auto-reply is preserved (and its trigger matching was verified against master's behavior, including plurals). Easy to restore if you want it back.

Verification

• yarn lint, yarn format:check, yarn typecheck, yarn test (50/50), yarn build — all green
• Smoke-ran both tsx dev mode and the compiled build with a fake token: both load all 19 commands
• Multi-agent adversarial review of the full diff vs master; all confirmed findings fixed (trigger-regex regression, presence-version regression, docs/gating mismatch)