Make p paste as children and P as siblings below

After trying part 8c, the user found p and P doing the same thing on the
canvas: both pasted a sibling below the node (P only differed in the
outline's order). Their pick: p pastes under the node, as its last
children, the way a adds a child; P pastes the next siblings, below it,
where o adds one. ]p went away, since p does what it did.

Inside a title p and P still paste after and before the text cursor, as
in vim: they're their own commands (paste-after, paste-before), and the
node ones are paste-as-child and paste-as-sibling.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

@@ -72,7 +72,7 @@ The plan for the everyday keys (all of them are in DESIGN.md):
 | `o` `O` `a`       | new sibling below / above, new child: type its title, Enter for the next one |
 | `i` or Enter      | edit the title (Esc: vim keys inside it, Esc again: done) |
 | `dd` `x`          | delete the branch / just the node (kept in a register, as in vim) |
-| `yy` `p` `P` `]p` | yank the branch / paste after, before, as a child (`dd` then `p` moves a branch) |
+| `yy` `p` `P`      | yank the branch / paste as children, or as siblings below (`dd` then `p` moves a branch) |
 | `"a` `"+`         | before `y`, `d` or `p`: register `a`; `"+` is the system clipboard (nodes go there as Markdown) |
 | `.`               | the last change again                              |
 | `>>` `<<`         | under the node above / out a level                 |