fix(opencode): merge session list to prevent stored sessions retrieval to overwrite active (new) session

[pointu-l]

With large session counts, the stored sessions retrieval is slower. Creating a session while a request is in-flight causes the UI to disappear when the stored sessions list replaces the store.

The session view depends on sync.session.get(route.sessionID) (index.tsx:114) rendered via <Show when={session()}> (index.tsx:962). When the store is replaced without the new session, session() becomes undefined and the Show renders nothing.

This fix merges the stored sessions with existing sessions instead of replacing, preserving newly-created sessions. Deduplication is covered by usage of Map.

Verification I did:

• With ~13k sessions, created a new session during an in-flight list request
• UI no longer disappears or blink when stored sessions response arrives in store

Fixes #10578

[github-actions]

Hey! Your PR title merge session list to prevent stored sessions retrieval to overwrite active (new) session doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found