Problem: toggle_interactive() had its condition inverted — it blocked
:CP interact on non-interactive problems while showing the message "This
problem is interactive", and passed through on interactive ones. The
panel guard in toggle_panel() was also missing a nil-check on
contest_data.index_map, which could crash if the index map was absent.
Solution: invert the toggle_interactive() guard to match the symmetrical
pattern in toggle_view(), fix the error message to say "not interactive",
and add the missing index_map guard. Also handle the stress panel type
in M.disable() so :CP stress can be toggled off.
Problem: the hooks API conflated distinct lifecycle scopes under a flat
table with inconsistent naming (setup_code, before_run, setup_io_input),
making it hard to reason about when each hook fires.
Solution: introduce two namespaces — hooks.setup.{contest,code,io} for
one-time initialization and hooks.on.{enter,run,debug} for recurring
events. hooks.setup.contest fires once when a contest dir is newly
created; hooks.on.enter is registered as a buffer-scoped BufEnter
autocmd and fires immediately after setup.code. The provisional buffer
setup_code callsite is removed as it ran on an unresolved temp buffer.
