Document solver permission levels in admin-add-solver

[mostronatorcoder]

Summary

Document the admin-add-solver protocol change introduced by MostroP2P/mostro#708.

text_message now supports optional solver permission suffixes:

• npub1... -> default read-write
• npub1...:read -> read
• npub1...:read-write -> read-write
• npub1...:write -> alias for read-write

Why

The Mostro implementation now distinguishes between:

• read-only solvers, which can take disputes and communicate with users
• read-write solvers, which can additionally execute admin-settle and admin-cancel

That changes the accepted payload format for admin-add-solver, so the protocol doc must reflect it.

Notes

No other wire-format changes were introduced in the Mostro PR. admin-settle and admin-cancel keep the same message shape; only authorization semantics changed there.

Related:

• Add read and read-write dispute solver permissions mostro#708

[grunch]

LGTM