[docs] README positioning update

[Sph3ricalPeter]

Priority: P1
Effort: ~1 day
AI required: No

Reposition prefetch's README around the Linear-first, AI-optional pitch.

Scope:

• Tagline: "Desktop git client for teams on Linear. Free, MIT, no AI subscription."
• Feature list grouped: Core git / Linear integration / Optional AI
• Comparison table vs GitKraken focused on pricing, BYOK without subscription gate, no credit metering
• Screenshots of Linear integration features once shipped
• "Why Linear-first?" section explaining the PR-less traceability gap

Acceptance criteria:

• New tagline and positioning
• Clear separation: AI-optional vs AI-required features
• Honest comparison table
• No marketing fluff — match the existing tone