Bump firmware to 4.4; add FLASHING.md runbook

Adds a step-by-step flashing guide (build, SWD first install, write protection,
OTA updates, recovery) and links it from the README. Bumps minor_version to 4.4
so the next release exercises a real over-the-air update from 4.3.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ci: grant release workflow contents:write for action-gh-release

The default GITHUB_TOKEN is read-only, so creating/updating the release (and
generating notes) failed with "Resource not accessible by integration". Add a
contents:write permission to the release job.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>