docs+refactor: polish documentation and consolidate duplicate code

refactor(core): move getDuplicateKey to models.js

chore: remove port probing console noise

fix(ui): preserve fallback editor state across picker flows

fix(diagnostics): support singular opencode provider keys

fix(models): support nested provider model ids

fix(ui): fallback model picker returns to editor after selection

fix(ui): keep fallback model picker open after adding models

fix(test): repair syntax error in fallback-picker-dedupe-test.js

fix(test): repair syntax error in fallback-ui-contract-test.js

fix(test): repair syntax error in fallback-ui-contract-test.js

fix(test): repair syntax error in fallback-ui-diff-test.js

fix(ui): remove duplicate calculateDiff function definition

fix(ui): define missing formatFallbackChangeDisplay function

feat(release): add strict upstream readiness gate