Fix: invalid macro syntax

[meelon-dev]

Is this resolving a feature or a bug?

This resolves a documentation bug: incorrect Rust attribute syntax in runtime/README.md. Attributes were written without the # prefix, which is invalid Rust and would not compile if copied.

Are there existing issue(s) that this PR would close?

No existing issue.

If this PR is not minimal (it could be split into multiple PRs), please explain why the issues are best resolved together.

The change is minimal and focused: all affected attribute examples in the runtime/README.md are fixed in one commit. Splitting would not provide any additional clarity.

Describe your changes.

Replaced incorrect attribute syntax in the README:

• [nexus_rt::private_input(x)] > #[nexus_rt::private_input(x)]
• [nexus_rt::custom_input] > #[nexus_rt::custom_input]
• [nexus_rt::custom_output] > #[nexus_rt::custom_output]

This ensures that documentation examples match valid Rust syntax and prevents compilation errors when copied.

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[slumber]

Thank you for the PR.

It's clear from the context how macros should be used.