Sell advisor: flag the weakest combo piece on a full board

JCA.weakest_link() scores each owned joker's total synergy against the
rest of the board (same capped scoring the shop advice uses) and returns
the lowest scorer — only when the board is full (that's when a cut
decision exists), has 3+ jokers, and one joker actually stands out
(uniform boards return nil). The owned-joker tooltip gains a red
'Weakest combo piece - a sell candidate' verdict line, suppressed in
learning mode like the other verdicts. Config toggle: sell_advisor.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

config.lua

@@ -4,6 +4,7 @@ return {
     pulse = true,          -- recommended shop jokers pulse periodically
     owned_tooltip = true,  -- show "Active combos" tooltip on owned jokers
     partner_highlight = true, -- hovering a joker pulses its owned partners
+    sell_advisor = true,   -- on a full board, flag the weakest combo piece
     threshold = 4,         -- total synergy score needed to recommend a card
     learning_mode = false, -- hide verdicts (pulse, "Strong pick!"), keep reasons
     touch_mode = true,     -- enlarge advice text/buttons for phone screens