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>

docs/credential-sharing.md

@@ -1,9 +1,49 @@
 # Credential Sharing OpenSpec
 
-Status: coming soon
+Status: reference implementation available (`@logicsrc/plugin-credential-sharing`)
 
 Slug: `credential-sharing`
 
+## Reference Implementation
+
+The spec below is implemented by `plugins/credential-sharing` and surfaced through
+`logicsrc credentials <command>`. All four first providers (`env`, `doppler`,
+`railway`, `github-secrets`) ship as provider adapters.
+
+```bash
+# List adapters and their capabilities (which can read values vs. write-only)
+logicsrc credentials providers
+
+# Inspect an endpoint — redacted key names + value fingerprints, never raw values
+logicsrc credentials inspect --provider env --path .env
+
+# Diff a source against a target without moving anything
+logicsrc credentials diff --from env --from-path .env --to railway \
+  --to-project <projectId> --to-config <environmentId>
+
+# Build a plan (stored under .logicsrc/credentials), then dry-run, then apply
+logicsrc credentials plan --from env --from-path .env --to doppler \
+  --to-project <project> --to-config <config>
+logicsrc credentials sync --plan <planId>            # dry-run (no writes)
+logicsrc credentials sync --plan <planId> --approve  # writes to the target
+
+# Audit and reverse a run (rollback emits a NEW plan)
+logicsrc credentials audit --run <runId> --format markdown
+logicsrc credentials rollback --run <runId>
+```
+
+SDK usage mirrors the spec via `createCredentialEngine()` from the plugin package.
+
+Implementation notes:
+- Value fingerprints are salted SHA-256 (truncated) so two endpoints can be diffed
+  without revealing values; they are equality/integrity markers, not secret storage.
+- `github-secrets` is write-only for values (GitHub never returns secret values), so
+  it cannot be a sync source or a value-restoring rollback target. Secret writes are
+  libsodium sealed-box encrypted against the repo/org/environment public key.
+- Rollback captures the target's prior values into a 0600 vault under `.logicsrc/`
+  (gitignored) — the only place raw values touch disk. Plans, runs, and audit records
+  contain fingerprints only.
+
 Credential Sharing is a LogicSRC OpenSpec for portable, auditable secret synchronization across local files and infrastructure providers. It is intended to replace closed, proprietary credential-sharing workflows with a provider-neutral contract.
 
 LogicSRC defines the open objects, CLI commands, SDK calls, TUI states, PWA states, provider adapter capabilities, and audit records. External products may consume this contract, but LogicSRC does not call out to product-specific commands.