docs: add public JSDoc to tools, skills, mcp and cross-platform (DOC-03)

[EmersonBraun]

What

Documents the public API of @agentskit/tools, @agentskit/skills, @agentskit/mcp and @agentskit/cross-platform (one commit per package) and regenerates the JSDoc coverage baseline once. Supersedes #1814.

Why

DOC-03: public exports need JSDoc.

How

• Documentation comments and patch changesets.
• The nested-ternary gate requires touched files to be clean, so two pre-existing nested ternaries in skills were flattened with equivalent code: compose.ts names the activate hook (activateAll), translator.ts moves the glossary IIFE into renderGlossaryData (same output string).
• Known residuals kept in the shrink-only baseline: 17 tools symbols re-exported from packages/integrations (documented at the origin by docs: document remaining public APIs (DOC-03 batch 4) #1816) and 6 cross-platform path aliases from the external pathe package.

Validation

• pnpm build ok; package lint ok; @agentskit/skills tests 444/444
• node scripts/check-jsdoc-coverage.mjs ok — 1673/1954 (85.6%); baseline: 141 entries removed, 0 added
• node scripts/check-public-api-snapshot.mjs ok; size-limit ok; nested-ternary gate ok