feat(credential-sharing): implement the Credential Sharing OpenSpec (M1-M3)

New @logicsrc/plugin-credential-sharing: a provider-neutral secret-sync engine
with env/.env, Doppler, Railway, and GitHub Secrets adapters behind one
CredentialProvider contract.

- engine: inspect -> diff -> plan -> approve -> sync -> rollback -> audit/export
- dry-run is the default for sync; --approve writes; destructive changes gated
- fingerprint-based diffs (salted SHA-256); raw values never printed or stored in
  plans/runs/audit; rollback pre-image kept in a 0600 .logicsrc vault (gitignored)
- github-secrets is write-only for values (sealed-box via libsodium), so it cannot
  be a sync source or value-restoring rollback target
- CLI: real `logicsrc credentials <providers|inspect|diff|plan|approve|sync|
  rollback|audit|export>` (replaces the prior stub)
- 4 JSON schemas registered in @logicsrc/validators
- flip logicsrc.com/credential-sharing band from coming-soon to available
- 37 tests pass; full env->env lifecycle verified; artifacts schema-validate

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -9,4 +9,5 @@ playwright-report
 test-results
 *.tsbuildinfo
 .commandboard
+.logicsrc
 supabase/.temp