Add post_id qualifier for like.create events

[mintlify]

Documents the new post_id filter qualifier on XAA like.create events, which narrows delivery to likes on a specific Post.

Changes to x-api/activity/introduction.mdx:

• Added post_id row to the filter qualifiers table (supported on like.create)
• Added a JSON example subscription filtering like.create by post_id

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Requested by: mrosinsky@x.ai via Slack
Mintlify session: slack_1789493235.142429_C09MQDT6BM4

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[mintlify]

Docs PR opened: #413

Added a Sep 15, 2026 changelog entry announcing the new post_id filter qualifier for like.create events on X Activity API.