C6 [CRIT]: unwind A13 over-fit on label create 422

[espadonne]

Summary

A13 ("friendly 409 on pre-seeded labels") taught label create to render a 422 as label "<name>" already exists; pass --force to overwrite. The audit caught the over-fit: every 422 — length, charset, color shape — is now reported as already-exists. Real validation failures are silently rewritten as uniqueness conflicts, pushing users toward --force for problems --force cannot fix.

C-audit reproducer (pre-fix):

shithub label create -R owner/r "   " --color abc123
# → label "   " already exists on owner/r; pass --force to overwrite
# (server actually said: 422 issues: label name length 1-50)

shithub label create -R owner/r "$(printf x%.0s {1..60})" --color abc123
# → label "xxxxx…" already exists; pass --force to overwrite
# (server actually said: 422 name too long)

Fix

pkg/cmd/label/create/create.go:isAlreadyExists:

• 409 → always uniqueness (clear status semantic)
• 422 → only if the server message contains "already taken", "already exists", or "duplicate"
• All other 422s fall through to the raw server error path

Server's actual uniqueness message (from shithub/.docs and direct API probing) is label name already taken on this repo; the substring check is robust to phrasing tweaks.

Test plan

• TestCreate409TreatedAsConflict — 409 maps to friendly error.
• TestCreateUniquenessConflictNoForce — 422 with "already taken" maps to friendly error.
• TestCreate422LengthErrorNotMisclassified — regression test for C6: 422 with "length 1-50" propagates the raw message; never mentions --force.
• TestCreateForceFallsBackToEdit — --force PATCH path still works on uniqueness conflicts.
• make ci green locally.