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

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