Add show_review_threads action to review_diff mappings #1477

Problem

When reviews.auto_show_threads = false, there is no default keymap to
manually open the thread panel at cursor in a review diff buffer. The
functionality exists — :Octo review thread calls
require("octo.reviews.thread-panel").show_review_threads(true) — but it is
not exposed as a mappable action.

Repro

Config:

require("octo").setup({
  reviews = { auto_show_threads = false },
  mappings = {
    review_diff = {
      show_review_threads = { lhs = "<localleader>vt", desc = "show threads" },
    },
  },
})

Expected: <localleader>vt opens the thread panel at cursor.

Actual: keymap is silently ignored.

Root Cause

utils.apply_mappings (lua/octo/utils.lua:1867) skips any entry whose
action key does not exist in the octo.mappings module:

and not M.is_blank(mappings[action])

lua/octo/mappings.lua defines next_thread, prev_thread,
resolve_thread, unresolve_thread, etc., but not
show_review_threads, so user config for it is dropped without warning.

Environment

• octo.nvim: master (as of 2026-04-16)
• Neovim: 0.11+