fix(github): compose channel event handlers

[iroiro147]

Summary

• add composable channel event helpers (afterDefault, beforeDefault, replaceDefault) to the public eve/channels surface
• make GitHub channel events compose after built-in handlers by default, so custom status handlers do not disable checkout, replies, or failure comments unless replacement is explicit
• document the GitHub event composition contract and add regression coverage for default reply, checkout preservation, and explicit replacement

Refs #623.

Tests

• pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/public/channels/github/githubChannel.test.ts
• pnpm --filter eve run typecheck
• pnpm exec oxfmt --check packages/eve/src/public/definitions/channel.ts packages/eve/src/public/channels/index.ts packages/eve/src/public/channels/github/githubChannel.ts packages/eve/src/public/channels/github/defaults.ts packages/eve/src/public/channels/github/githubChannel.test.ts docs/channels/github.mdx .changeset/channel-event-composition.md
• pnpm exec oxlint --deny-warnings packages/eve/src/public/definitions/channel.ts packages/eve/src/public/channels/index.ts packages/eve/src/public/channels/github/githubChannel.ts packages/eve/src/public/channels/github/defaults.ts packages/eve/src/public/channels/github/githubChannel.test.ts
• pnpm docs:check
• pnpm guard:invariants
• git diff --check
• pnpm changeset status --since origin/main

[vercel]

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[iroiro147]

Rebased this onto current main and pushed signed head 7ed7a87c. I also completed the public composition contract: minor semver, direct coverage for bare/after/before/replace ordering, missing defaults, awaited sequencing, and exception short-circuiting; user-facing docs; JSDoc on the exported API; and public portability coverage for GitHubChannelEventHandlers. Verified under Node 24 with 47 focused unit tests, the 8-case public portability scenario, package typecheck, all documentation checks, invariant checks, and git diff --check. Fresh CI is running.

[iroiro147]

Rebased onto current main (was CONFLICTING). Resolved the import conflict in packages/eve/src/public/channels/github/githubChannel.ts (HEAD's readNonEmptyString import + this branch's composeChannelEvents/ChannelEventConfigMap import — both retained). Verification on the rebased head: typecheck ✓ and the github-channel + channel-definition unit tests pass (48/48).