mcp(docs[changes]): reframe respawn shell rename as pre-release decision

Move the ``shell_command -> shell`` bullet out of "Breaking changes"
into "API decisions (pre-release)". The rename happened entirely on
this branch and never reached ``origin/main``, so there is no shipped
contract to break — calling it a *breaking change* in the changelog
miscommunicates state to readers tracking releases. The remaining
prose still records why the name landed at ``shell`` (alignment with
``split_window`` + upstream ``Pane.respawn`` on the ``tmux-parity``
branch); the speculative "renaming after caching is a protocol break"
sentence is dropped — that's a future-state risk note, not a
description of *this* release.

Author: tony

CHANGES

@@ -83,14 +83,14 @@ _Notes on upcoming releases will be added here_
   table picks up a `respawn-pane` row showing the unusual mutating-tier
   + `destructiveHint=true` + `idempotentHint=false` combination.
 
-### Breaking changes
+### API decisions (pre-release)
 
 - {tooliconl}`respawn-pane` parameter ``shell_command`` is renamed to
-  ``shell`` to align with {tooliconl}`split-window` and the eventual
-  upstream ``Pane.respawn(shell=)`` API on libtmux's ``tmux-parity``
-  branch. Renamed before agents cache the schema; renaming after
-  caching is an MCP protocol break. No compatibility alias — this is
-  a pre-release library.
+  ``shell`` to align with {tooliconl}`split-window` and the upstream
+  ``Pane.respawn(shell=)`` signature on libtmux's ``tmux-parity``
+  branch. Settled before the tool reached ``origin/main`` so no
+  compatibility alias is shipped — alignment is the load-bearing
+  reason, not a fix to a previously released name.
 
 ### Refactor