fix(taste): rejected preferences must carry rejection-strength confidence

[jbetala7]

Repro / observed problem

In bin/gstack-taste-update, every preference in a rejected bucket is permanently stored with confidence: 0, regardless of how many times the value is rejected. On upstream main (19770ea):

$ gstack-taste-update rejected v1 --reason "colors: crimson"   # x3
$ gstack-taste-update rejected vx --reason "colors: beige"     # x1
$ gstack-taste-update show
[colors]
  rejected:
    crimson — conf 0.00 (+0/-3)
    beige — conf 0.00 (+0/-1)

Root cause

bumpPref() computed confidence as approval rate for both buckets:

const total = entry.approved_count + entry.rejected_count;
entry.confidence = entry.approved_count / (total + 1);

The approved and rejected lists are disjoint, so a rejected-bucket entry only ever gets rejected_count incremented — approved_count stays 0 and confidence is always 0. That broke three things:

1. show prints conf 0.00 for every rejection.
2. show ranks rejections by confidence * rejected_count = 0 for all, so the top-3 is insertion order, not the strongest rejections.
3. The taste-drift warning (opp.confidence >= 0.6) is unreachable through real CLI use. The existing test only passed it by hand-seeding confidence: 0.8 via writeProfile, with a comment admitting the natural value is 0/5.

Fix

Compute confidence from this bucket's own count:

const total = entry.approved_count + entry.rejected_count;
const ownCount = action === 'approved' ? entry.approved_count : entry.rejected_count;
entry.confidence = ownCount / (total + 1);

Approved-bucket confidence is unchanged (there total === approved_count, so approved_count/(total+1) is identical). Rejected entries now reflect rejection strength, so crimson (3 rejections) → 0.75 and beige (1) → 0.50, the ranking orders by strength, and the drift warning fires after enough real rejections.

Testing

bun test test/taste-engine.test.ts — 22 pass (19 existing + 3 new). Added:

• repeated rejections raise rejected confidence toward 1 (5 rejections → 5/6)
• show ranks rejections by strength, not insertion order
• drift warning fires from real CLI rejections (no seeding)

All three fail on main and pass with this change. Verified git diff --check clean and git merge-tree --write-tree HEAD origin/main merges cleanly.

Fixes #1776