release notes: mac app theme, watchdog, gates/MCP wave, fresh-Mac upgrade note

[m4ttheweric]

Refreshes the v2.9.0 RELEASE_NOTES draft over settled main (1fa920b). Adds the missing mac app theme (#282/#283/#286/#287/#292/#238), the watchdog line and its follow-ups (#301/#304/#305/#296), the post-draft gates/MCP wave (#275..#299), setup additions (#265/#294/#302/#285), the git-core library (#303), and the required upgrade note stating plainly that v2.8.0 does not install on a fresh Mac and why (MAT-386 checklist item).

Verified: no em/en dashes, no employer terms, repo-purity.sh ok, all 26 cited PR numbers exist in v2.8.0..origin/main.

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• RELEASE_NOTES.md

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