Document operator vs sender-only action sets

[Jaydbrown]

List exactly which entry points an Operator may call (pause, cancel, clawback, top_up, extend_duration) vs sender-only, in the README and the contract doc comments.

[ZacLou]

I'd like to work on this issue. I have relevant experience with this tech stack and can deliver a clean, well-tested PR. I can submit within 1-2 days.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@ZacLou's PR #516 was approved and merged by @Jaydbrown.

🏆 @ZacLou: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (503 total points). Track your full progress on GrantFox.

👏 Great work, @ZacLou! Keep contributing to conduit-protocol.