feat(picker_ui): add opts.on_submit hook for custom selection action

[gustav-fff]

Closes #605

Root cause

Picker UI hardcodes :edit (or split/vsplit/tab) on selection at lua/fff/picker_ui/picker_ui.lua:498. The only extension point — select.select_window (lua/fff/conf.lua:65) — picks the target window but cannot replace the open action itself. Use-cases like "open the directory of the selected file in oil.nvim" have no hook.

Fix

Add opts.on_submit to find_files(opts). When set, it runs in place of the default open action after the picker closes. Receives the item plus a context table (action, abs path, relative path, location, query, mode). Query history tracking still runs. No behavior change when on_submit is unset.

Signature: fun(item, ctx) where ctx = { action, path, relative_path, location, query, mode }.

Steps to reproduce

Pre-fix on main: there is no way to intercept selection. After the patch:

require('fff').find_files({
  title = 'Oil',
  on_submit = function(item, ctx)
    local dir = ctx.path:match('^(.*)[/\\\\][^/\\\\]*$')
    require('oil').toggle_float(dir)
  end,
})

Expected: oil opens at the directory containing the selected file. Actual (pre-fix): the file is :edit-ed; reporter has no hook to override.

How verified

Manual review of M.select flow: on_submit runs inside the same vim.schedule after M.close(), so picker teardown completes before the callback fires (same ordering as the default :edit path). When on_submit is unset the original path is taken verbatim.

Did not run a Neovim-level e2e — flagging for maintainer to sanity-check the oil integration interactively.

Automated triage via Gustav. Honk-Honk 🪿

[dmtrKovalenko]

@gustav-fff I remember there was another related issue please find it and link

[gustav-fff]

[triage-bot] DIRECTED: related issue is #247 ("API: more structure and pass callback to all provided APIs") — same theme of exposing callback hooks on the public API. This PR addresses the selection-callback slice; #247 is broader (callbacks across all APIs) and remains open.

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