taste profile: rejected preferences always have confidence 0 (broken `show` ranking + dead drift warning)

[jbetala7]

Observed problem

In bin/gstack-taste-update, every preference in a rejected bucket is permanently stored with confidence: 0. This makes the show output useless for rejections and silently disables the taste-drift conflict warning.

Root cause

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

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

A rejected-bucket entry only ever gets rejected_count incremented (the approved and rejected lists are disjoint — a value approved goes in approved, rejected goes in rejected). So approved_count stays 0 and confidence = 0 / (rejected_count + 1) = 0 no matter how many times the value is rejected.

Downstream effects:

1. show display prints conf 0.00 for every rejected preference (bin/gstack-taste-update cmdShow).
2. show ranking sorts rejections by confidence * rejected_count, which is 0 for all of them, so .slice(0, 3) returns insertion order instead of the strongest rejections.
3. Taste-drift warning at if (opp && opp.approved_count + opp.rejected_count >= 3 && opp.confidence >= 0.6) is unreachable through real CLI use, because a rejected entry's confidence can never exceed 0. (The existing test test/taste-engine.test.ts only passes it by hand-seeding confidence: 0.8 via writeProfile, with a comment acknowledging the natural value is 0/5.)

Current behavior on upstream main (19770ea)

Reproduced with a temp state dir + git repo:

$ 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)

Both confidences are 0, and the profile JSON stores "confidence": 0 for both. A value rejected 3 times ranks no differently from one rejected once.

Expected behavior

A rejected preference's confidence should reflect rejection strength (symmetric with the approved bucket): rejected_count / (total + 1). After the fix, crimson (3 rejections) → conf 0.75, beige (1 rejection) → conf 0.50, the show ranking orders by strength, and the drift warning fires naturally after enough real rejections.

Duplicate searches performed

• open issues / PRs: taste, in:title taste, confidence rejected taste → no match
• recently closed PRs touching bin/gstack-taste-update → only the originating feature PR feat(v1.3.0.0): open agents learnings + cross-model benchmark skill #1040 (v1.3.0.0); file untouched since
• no open PR touches bin/gstack-taste-update

Candidate fix shape

In bumpPref(), 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);

This leaves approved-bucket confidence identical (for approved entries total === approved_count) and only fixes the rejected case. Add regression coverage for rejected confidence, the show ranking, and the drift warning firing from real CLI rejections.

[jbetala7]

PR: #1777