feat(channels): export helper APIs

[iroiro147]

Summary

• export defaultInputRequestedHandler from eve/channels/slack
• export callGitHubApi from eve/channels/github and verifyLinearRequest from eve/channels/linear
• document the helper usage and extend public API portability coverage for the new imports

Closes #351.

Tests

• pnpm --filter eve exec vitest run --config vitest.scenario.config.ts test/scenarios/public-api-portability.scenario.test.ts
• pnpm --filter eve run typecheck
• pnpm docs:check
• pnpm guard:invariants
• pnpm exec oxfmt --check packages/eve/src/public/channels/slack/index.ts packages/eve/src/public/channels/github/index.ts packages/eve/src/public/channels/linear/index.ts packages/eve/src/internal/testing/scenario-apps/slack-route-portability.ts packages/eve/src/internal/testing/scenario-apps/github-route-portability.ts packages/eve/src/internal/testing/scenario-apps/linear-route-portability.ts packages/eve/src/internal/testing/scenario-apps/index.ts packages/eve/test/scenarios/public-api-portability.scenario.test.ts docs/channels/slack.mdx docs/channels/github.mdx docs/channels/linear.mdx .changeset/channel-helper-exports.md
• pnpm --filter eve exec oxlint src/public/channels/slack/index.ts src/public/channels/github/index.ts src/public/channels/linear/index.ts src/internal/testing/scenario-apps/slack-route-portability.ts src/internal/testing/scenario-apps/github-route-portability.ts src/internal/testing/scenario-apps/linear-route-portability.ts src/internal/testing/scenario-apps/index.ts test/scenarios/public-api-portability.scenario.test.ts --deny-warnings
• pnpm changeset status --since origin/main
• git diff --cached --check

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