Add missing `permission-discussions` input to action.yml

[salmanmkc]

Problem

The action.yml declares ~40+ permission-* inputs in its generated permissions block, but permission-discussions is missing. It should appear between permission-deployments and permission-email-addresses (alphabetically), but was omitted from the generated list.

Current behavior

Because permission-discussions is not declared in action.yml, any workflow that passes permission-discussions: read or permission-discussions: write sees an "Unexpected input" warning in the Actions logs:

Warning: Unexpected input(s) 'permission-discussions', ...

Despite the warning, the permission does work at runtime — the action reads all INPUT_PERMISSION-* environment variables from process.env (via get-permissions-from-inputs.js), so the value is forwarded to the GitHub API regardless. The issue is purely cosmetic but confusing for users.

Why this matters now

github/gh-aw#25709 restores the compiler-side mapping that passes permission-discussions: write to create-github-app-token for workflows that interact with GitHub Discussions. With that PR merged, tokens are correctly scoped, but every such workflow run will emit the "Unexpected input" warning until this action declares the input.

Proposed fix

Add permission-discussions to the generated permissions block in action.yml, between permission-deployments and permission-email-addresses:

  permission-discussions:
    description: "The level of permission to grant the access token for discussions. Can be set to 'read' or 'write'."

This is consistent with how all other permission-* inputs are declared. No code changes are needed — only the action.yml input declaration.