README: renumber steps 1-8 + Final, merge bonus Ghostty+Arc into Step 2

Step renumbering:
- Step 1 - Claude (was 'Get Claude Running')
- Step 2 - Bonus Software (merges old Bonus #1 Ghostty + Bonus #2 Arc into one step w/ H3 subsections)
- Step 3 - Developer & Utility Tools (was Step 2)
- Step 4 - FidgetFlo + Context Hub (was Step 3)
- Step 5 - Productivity Tools (was Step 6)
- Step 6 - Telegram (was Step 8)
- Step 7 - GitHub (was Step 10 'Developer Tools', renamed + moved up + absorbs /gitfix from Final)
- Step 8 - Safety Check (was Step 9)
- Final Step - Status Line (stripped /gitfix, now Step 7)

Updates:
- Quick Navigation: fully renumbered rows + anchors
- How It Works: fully renumbered bullets + descriptions
- Step 2 body: merged Ghostty + Arc with H3 subsection headers
- Step 7 body: rewrote to cover both GitHub MCP + /gitfix skill
- Final Step body: removed /gitfix references (moved to Step 7)
- Install URLs in Step 2 point to /step-2/ghostty-install.sh and /step-2/arc-install.sh (folders to be renamed next commit)

Author: fidgetcoding