chore: merge dev into v2

[opencode-agent]

Summary

• continue from the retained prior merge checkpoint and merge latest v2 (7211c993) followed by latest dev (95636bd3)
• preserve V2-owned Core/Server/CLI/CodeMode/TUI architecture and legacy package deletions
• take packages/app, packages/desktop, and packages/session-ui completely from dev as the source of truth for Desktop/Web V2 compatibility
• adapt dev App event/types to the current workspace @opencode-ai/client contracts
• vendor dev’s @opencode-ai/sdk 1.18.8 build as packages/app/vendor/opencode-ai-sdk-1.18.8-dev.tgz and remove the vendored client snapshot
• preserve dev’s non-conflicting Core repository/reference fixes
• regenerate bun.lock from the final manifests

Validation

• packages/app: bun typecheck
• packages/session-ui: bun typecheck; 76 tests pass
• packages/desktop: bun typecheck
• packages/ui: bun typecheck
• packages/core: bun typecheck; 14 focused repository/reference tests pass
• 20 focused current-client adapter/session projection tests pass
• App full unit suite: 684 pass, 1 pre-existing App i18n parity failure (dialog.provider.custom.label, dialog.model.unpaid.viewMoreProviders, and three session.header.reveal.* keys exist only in English on current dev/V2)
• Production benchmark unavailable: the Host production build previously terminated during chunk rendering before metrics were produced

Review notes

• Conflict and commit audit was reviewed in the originating Slack thread.
• Do not merge without explicit approval from @Brendonovich.

Requested by: @rekram1-node (Aiden Cline via Slack)

• Nix hashes need recomputation after the final SDK/client lockfile swap. The bot installation cannot dispatch the nix-hashes workflow on this branch (actions:write is not granted).