fix: systematic context guards and full panel lifecycle (#317)

## Problem

The `CONTEST_ACTIONS` list in `handle_command` was a manually maintained
parallel of `ACTIONS` that had already drifted (`pick` was incorrectly
included, breaking `:CP pick` cold). `navigate_problem` only cancelled
the `'run'` panel on `next`/`prev`, leaving interactive and stress
terminals alive and in-flight `run_io_view` callbacks unguarded.
`pick` and `cache` also appeared twice in tab-completion when a contest
was active.

## Solution

Replace `CONTEST_ACTIONS` with a `requires_context` field on
`ParsedCommand`,
set at each parse-site in `parse_command` so the semantics live with the
action definition. Expand `navigate_problem` to call `cancel_io_view()`
and
dispatch `cancel_interactive`/`stress.cancel` for all panel types,
mirroring
the contest-switch logic. Filter already-present `pick` and `cache` from
the
context-conditional completion candidates.

plugin/cp.lua

@@ -44,7 +44,12 @@ end, {
       table.insert(candidates, 'cache')
       table.insert(candidates, 'pick')
       if platform and contest_id then
-        vim.list_extend(candidates, actions)
+        vim.list_extend(
+          candidates,
+          vim.tbl_filter(function(a)
+            return a ~= 'pick' and a ~= 'cache'
+          end, actions)
+        )
         local cache = require('cp.cache')
         cache.load()
         local contest_data = cache.get_contest_data(platform, contest_id)