docs: rewrite the README editing section for the unified save model

The README still described the old per-field push commands and the org-sync hook mode, both removed in save-model-v2. I rewrote the editing section around the one write path: you edit in the buffer, save the ticket, and Pearl reconciles each changed field against the remote. It now leads with pearl-save-issue / pearl-save-all, spells out how each field is edited (the cookie for priority, a picker for state/assignee/labels, hand-editing for text), explains that picking a constrained field marks it changed rather than pushing, and describes the conflict gate for both free-text and atomic fields.

The old "Org TODO sync" section drops the removed enable/disable hooks and keeps the state-to-keyword mapping as render config, with pearl-edit-state as the way to change state. The intro, the feature list, and the use-package and prefix-key examples lose their stale references. I kept the keyword-cycle-to-state and keyword-derivation work named as planned follow-ups rather than claiming they work, since they're not in yet.

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