feat(lua): inline input completers with a bundled @-files source

[Firaenix]

Independent of the other open PRs; based directly on main.

Typing a registered trigger char at a word boundary in the prompt opens a completion popup fed by a Lua handler:

maki.api.register_input_completer({
  trigger = "#",
  name = "issues",
  handler = function(query)
    return { { label = "#123 fix the thing", insert = "https://...", detail = "author" } }
  end,
})

The handler is re-queried on every keystroke and answers over a channel, so a slow source (a CLI call, an HTTP API) never blocks typing; the popup shows searching… until the first reply, no matches on an empty one, and closes on a handler error. Up/Down navigate, Tab/Enter replace the trigger and query with the selected item's insert text (defaulting to its label), Esc dismisses and keeps the literal text until the trigger context changes. Stale replies are discarded by generation, and /reload clears a plugin's completers.

A bundled file_mention plugin binds @ to project file paths (gitignore-aware, most recently modified first), so @ma<Tab> inserts src/main.rs out of the box.

Also adds maki.ui.insert_input(text), letting plugins put text into the prompt at the cursor without submitting it.

Covered by handler round-trip and registration tests on the Lua side, popup unit tests (trigger detection, re-query, accept, dismiss, failure), and app-level tests for accept-into-input and plain submit.