ReleaseProcessOverview.md: fix release stream naming notation -- the patch digit is always present, not optional.
Confirmed against what we actually did this session: the GA release
stream is r26.1.0 (not r26.1), so the doc's own '[.<P>]' optional-patch
notation was inconsistent with its own example. Now r<YYYY>.<N>.<P> with
an explicit note that <P> is always present (0 for GA).
Agent: Claude Code, Model: Claude Sonnet 5 (claude-sonnet-5), operating as bot_Claude_Anthropic.