Recycle brief if validation expands into multiple steps

[yjunechoe]

This PR aims to fix #478 by recycling brief when possible.

# Single string `brief` recycles
create_agent(small_table) |> 
  col_exists(c("a", "b"), brief = "one") |> 
  el("validation_set") |> 
  el("brief")
#> [1] "one" "one"

# Multi-length `brief` applies if exactly matched in number of steps produced
create_agent(small_table) |> 
  col_exists(c("a", "b"), brief = c("one", "two")) |> 
  el("validation_set") |> 
  el("brief")
#> [1] "one" "two"

# Errors if not matched in length (columns x segments produces 6 steps here)
create_agent(small_table) |> 
  col_vals_equal(
    c("a", "b"), 0,
    segments = vars(f),
    brief = c("one", "two")
  )
#> Error in `resolve_briefs()`:
#> ! `brief` must be length 1 or 6, not 2

This is a draft PR until I:

• Figure out the right level of abstraction for the resolve_briefs() helper - namely, should it stand on its own (as current) or wrap over the default autobrief behavior, to host all brief-related logic? (will move forward with a unified resolve_briefs() function)
• Do the necessary copy-pastes for all user-facing validation functions
• Write (batch?) tests

Out of scope but worth considering for afterwards is whether we should support glue sytnax here as well.