polish README hero: vertical gate stack, fix value-line collision

[azalio]

Follow-up polish to the README hero (shipped in #383).

Redesigns the hero's right rail to fix a layout collision and improve legibility:

• Before: the right rail was a cramped 2-row mini-loop (SPEC→PLAN→CODE / REVIEW→LEARN→next run) plus a dashed return arc, starting at x=740. The value line "You approve the plan before the model writes a line of code." (~715 SVG units) crowded that rail boundary.
• After: a clean vertical gate stack (SPEC, PLAN, TEST, CODE, REVIEW, LEARN) on a 248-unit rail, separated from the title block by a divider. PLAN stays the amber approval gate (✓ you approve), LEARN the green loop-back (↻ next run).

Font-metric verified (DejaVu, which is wider than the rendered -apple-system, so conservative):

• value line 2: 715 ≤ 770 available
• title: 544 ≤ 772 available
• gate-row labels fit their boxes with headroom

No README.md change — the hero alt text already describes the loop accurately. Only assets/readme/hero.svg is touched.

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/readme-beautify

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