docs(integrations): document public API

[EmersonBraun]

What

• Document the 84 undocumented root entry point symbols in @agentskit/integrations.
• Shrink the coverage baseline for the root entry point and declarations that re-export these symbols.

Why

• Help consumers understand integration descriptors, authentication types, registry operations, HTTP configuration, and supported service catalog entries from generated declarations.

How

• Add concise JSDoc to the root API declarations and service descriptors after reviewing their implementations.
• Add the required patch changeset. No runtime logic or public signatures changed.

Validation

• pnpm --filter @agentskit/integrations lint
• pnpm --filter @agentskit/integrations test (24 files, 157 tests passed)
• pnpm --filter @agentskit/integrations build
• Fresh workspace declaration build followed by node scripts/check-jsdoc-coverage.mjs --update (114/116 symbols documented; the 2 remaining symbols are in ./testing)
• pnpm check:public-api-snapshot (24 packages, 74 subpaths, 1,955 symbols; unchanged)
• Repository pre-push quality gates passed.

[EmersonBraun]

Superseded by #1816, which consolidates the integrations root and testing subpath documentation with the remaining batch packages.