docs: add Typefully draft URL trigger

[linuz90]

Typefully Agent Skill instructions now explicitly recognize dropped Typefully draft URLs and map their query params to the existing drafts:get command, so local agents have the trigger text they need without previewing deep into the skill body.

Refs TYP-7308

Summary

• Draft URL trigger: updates the skill frontmatter and top-level guidance to mention https://typefully.com/?a=<social_set_id>&d=<draft_id> as a direct trigger for the Typefully skill.
• Command mapping: documents that a maps to social_set_id, d maps to draft_id, and ?d=<draft_id> can use the configured default social set with --use-default.
• Changelog: records the new trigger behavior under the 2026-07-09 changelog section.

Related Links

• Linear: TYP-7308
• Frontend discovery PR: https://github.com/typefully/typefully-frontend/pull/3712

Validation

• npm test

Manual QA

• Ask a local agent about https://typefully.com/?a=123&d=456 → it should choose the Typefully skill and propose ./scripts/typefully.js drafts:get 123 456.

After Merge

• Merge before or alongside the frontend discovery PR so Typefully's skill discovery description and raw skill artifact stay aligned.