Plugin writes failed whenever the workspace read-only flag was set. The
Inspect tab sets it too, although it only hides the editing tools, so an
MCP agent could not edit while the user was inspecting. A user who
became a viewer had the flag cleared, so plugin writes did not throw and
silently did nothing.
Writes now fail on what makes them unsafe, not on the flag: failing
saves, a user who cannot edit, a version preview or a lost render
context. The error says which one, and the MCP instructions tell the
agent which of them pass by waiting.
A read-only workspace (saves failing, a user who cannot edit, a version
preview) only hid the editing tools: plugins, the MCP among them, still
changed shapes, and those edits were never saved. Every plugin write now
throws a read-only error, whatever the throwValidationErrors flag, the
same way a write to a removed shape does.
`check-permission` with a write permission runs the new `check-writable`,
which covers most writers. Writers without a permission check (most
context methods such as create*, group, align or flatten, the z-order
methods, applyTypography on a text range, removing a ruler guide) call it
themselves, and shape setters get it through `check-shape-writable`.
Reads, comments and undo blocks keep working, so a block opened before a
lock still closes.
The MCP instructions tell the agent what the read-only error means.
