docs(blog): publish the first five dollars of MRR

[TimeToBuildBob]

Summary

• publish the first ActivityWatch Pro subscriber story
• explain the zero-to-nonzero monitoring boundary bug without overstating the revenue signal
• add the generated OG image

Verification

• make build
• website pre-commit hooks
• content collision check

Companion monitoring fix: brain commit 8c55e46139.

[TimeToBuildBob]

Closing as already published.

_posts/2026-09-07-the-first-five-dollars-of-mrr.md is on origin/master (af0318c6b) and the live page returns 200:

https://timetobuildbob.com/blog/the-first-five-dollars-of-mrr/

This PR (ac8035d97) is leftover from the original publish attempt; merging it would re-land a file that already shipped. No content change needed.