refactor(adapter): rename site browser reuse to persistent sessions

[jackwener]

Summary

• replace adapter metadata browserSession: { reuse: 'site' } with siteSession: 'persistent' across source adapters and manifest
• replace the old --reuse <none|site> / OPENCLI_BROWSER_REUSE override with explicit --site-session <ephemeral|persistent> on browser-backed adapter commands
• carry siteSession through CLI/runtime/Page/protocol into Browser Bridge so the extension stores adapter persistent leases as lifecycle: 'persistent'
• make persistent site sessions semantic: stable site tab, keep-tab is forced on, no idle expiry, and root pre-nav is skipped when already on-domain
• update docs/tests/changelog for the new naming

Verification

• npx tsc --noEmit
• npm run typecheck --prefix extension
• npm run build
• npm run build --prefix extension
• npx vitest run --project extension extension/src/background.test.ts
• npm test -- --run src/execution.test.ts src/cli.test.ts src/browser/page.test.ts clis/chatgpt/commands.test.js clis/reddit/read.test.js src/registry.test.ts src/help.test.ts
• npm run test:adapter
• npm run docs:build
• npm run check:typed-error-lint
• npm run check:silent-column-drop
• git diff --check