docs: instruct linking Linear issues when opening PRs (#1604)

## Description

Adds a rule to `CLAUDE.md` instructing agents to use the Linear MCP (if
available) when opening a new pull request — either linking to related
existing issues or creating a new issue from the PR description.

Linked issue:
[SDK-267](https://linear.app/e2b/issue/SDK-267/claudemd-require-linear-issue-linking-when-opening-prs)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: mishushakov

CLAUDE.md

@@ -5,5 +5,6 @@ To re-generate the API client run `make codegen` in the repository root when mod
 Create or update tests covering affected codepaths and run them using `pnpm run test`.
 Generate a changeset when updating packages/cli, packages/js-sdk, packages/python-sdk with `pnpm changeset` in the repository root.
 When creating a pull request, add usage examples for user-facing changes to the PR description.
+When opening a new pull request, use the Linear MCP if available to link to related issues, or create a new issue from the PR description.
 Keep PR descriptions up-to-date with changes.
 Default credentials are stored in .env.local in the repository root or inside ~/.e2b/config.json.