fix: preserve config values containing =

[euyua9]

Summary

• preserve everything after the first = when parsing ai config set KEY=value
• keep rejecting malformed assignments that do not include a key before =

Repro

ai config set OPENAI_API_ENDPOINT=https://host/v1?api-key=abc==&deployment=x

Before this change, the stored value was truncated to https://host/v1?api-key.

Validation

• npm run typecheck
• npm run build
• HOME=$(mktemp -d) node dist/cli.mjs config set OPENAI_API_ENDPOINT='https://host/v1?api-key=abc==&deployment=x'
• confirmed ~/.ai-shell now keeps the full endpoint value