docs/agent-portability.md: references non-existent .claude/instructions.md

[nanaubusiness]

In docs/agent-portability.md line 26, the CodeWhale row says:

...also reads `.claude/instructions.md` as fallbacks.

But the file .claude/instructions.md does not exist anywhere in the repository. A user relying on this fallback will not find the promised behavior.

[nanaubusiness]

Already fixed in PR #370.