feat(typefully): add LinkedIn first comment support to drafts

[frankdilo]

Summary

Adds LinkedIn first comment support to drafts:create and drafts:update — the common "link in the first comment" pattern. Pairs with the API v2 change in typefully/typefully-backend#1770.

./scripts/typefully.js drafts:create --platform linkedin --text "Big launch today!" --linkedin-first-comment "Full details: https://example.com/launch"

CLI behavior

• New flag --linkedin-first-comment <text> on drafts:create/drafts:update (and the create-draft/update-draft aliases). Flag aliases: --linkedin_first_comment, --first-comment, --first_comment, with conflict detection when several are passed with different values.
• Sent to the API as platforms.linkedin.settings.first_comment. LinkedIn-only: using it without linkedin in the target platforms errors.
• On update:
  • literal null clears the comment;
  • omitting the flag preserves the existing comment — the CLI only sends settings when the flag is passed, and the API preserves the first comment when settings are omitted;
  • the flag alone (no --text) works as a settings-only update: the CLI re-sends the draft's existing LinkedIn posts, mirroring the X metadata-only update path.

Docs & tests

• SKILL.md: flags table, common actions row, LinkedIn-only flags pointer, last-updated bump.
• references/platforms/linkedin.md: new "First comment" section.
• CHANGELOG.md: user-facing entry under 2026-07-10.
• 6 new tests in tests/drafts.test.js (create payload, platform validation, settings-only update, settings omitted on text-only update, null clear, conflicting flags); full suite passes (122 tests).

Merge note: requires typefully/typefully-backend#1770 to be deployed first — the current API schema rejects platforms.linkedin.settings.first_comment with 422.