MCP OAuth prompt hardcodes opencode binary name

When an MCP server requires OAuth, opencode surfaces two prompts telling the user to run opencode mcp auth <name>:

• a toast published from packages/opencode/src/mcp/index.ts when connect fails with an auth error
• the "Needs authentication" line in the /mcp status dialog (packages/tui/src/component/dialog-status.tsx)

The opencode prefix is a literal string. Downstream distributions or wrappers that invoke the same binary under a different name give users a command that isn't on their $PATH.

Proposal: honor an OPENCODE_CLI_NAME env var (following the existing OPENCODE_* convention) and interpolate it into both prompts. Default "opencode" — no change for normal installs.

Repro: any OAuth-requiring MCP server (e.g. Sentry). The toast says Server "sentry" requires authentication. Run: opencode mcp auth sentry regardless of how the binary was invoked.

Happy to send the PR (~4 lines).