docs: cut the README to an operator front door

[sanlee-ys]

Why

The README was 517 lines and 4,221 words. Most of that was the same honesty speech after every command. A stranger could not say what the repo does after the first screen.

What changed

README.md is now 223 lines and about 1,000 words. Commands, pictures, and claims stay. Sermons leave.

Kept:

• four install paths (source, scoop, direct download, winget-not-yet)
• both hero pictures (council and HUD), same files the image tests emit
• the three statusline JSON blocks (doctor points at this section)
• \xattr -d\ outside the main \curl\ block (PARITY.md)
• snapshot meaning table, honest-gauge rule, fixture rule

Moved, not deleted:

• v1 cut argument → already in \docs/design.md\ §1
• signing and Gatekeeper walk → already in \SECURITY.md\
• scoop and winget argument → already in \packaging/\
• council detail → already in \docs/council.md\
• snapshot schema and events boundary → already in \docs/design.md\ §7.22 / §7.21

Verify

No Go file changed. No test reads \README.md. Image tests compare the SVG files, which this PR does not touch.

CI on this PR is the gate.

Merge

This is public front-door copy. Do not merge until you read the page aloud and accept it as the specimen.