feat: append-only version history + bliss CLI ergonomics

Snapshot the code-bearing fields of routes (verb/path/handler), templates
(name/content/test_object), and db libraries (name/library) on every
content-changing save, so nothing edited is ever lost. Recorded in a new
`versions` table via recordVersion() hooked into the create/update functions
in db.js (the single write choke point), deduped against the latest snapshot
so no-op saves, error-flag-only route writes, and server-restart WS
re-bootstraps don't pile up. The live row stays current; old snapshots are the
undo trail. Read-only, no revert UI yet.

- migrations/004_add_versions.sql: versions table + indexes
- db.js: recordVersion/getVersions/getVersion + create/update hooks
- plumbing.js: /versions/:id and per-entity .../versions list endpoints
- bliss: `versions <type> <sid> <id>` and `version <id>` reads;
  `update-settings` (edit route_prefix/head_injection, preserving the
  untouched field); `use <url>` sticky target persisted to ~/.bliss/target
  so BLISS_URL needn't be re-exported each call
- client.js: target-file precedence (BLISS_URL > ~/.bliss/target > localhost)
- SKILL.md: document the above

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

plumbing.js

@@ -72,6 +72,27 @@ router.get(
   json((req) => model.getFilesForStruct(req.params.id)),
 );
 
+// Version history (newest first) for a route handler, template, or db library.
+// Each row is a summary; fetch /versions/:versionId for the full snapshot.
+router.get(
+  "/structures/:id/routes/:routeId/versions",
+  json((req) => model.getVersions("route", req.params.routeId)),
+);
+router.get(
+  "/structures/:id/templates/:templateId/versions",
+  json((req) => model.getVersions("template", req.params.templateId)),
+);
+router.get(
+  "/structures/:id/dbs/:dbId/versions",
+  json((req) => model.getVersions("db", req.params.dbId)),
+);
+
+// One version, including its full snapshot (the versioned fields).
+router.get(
+  "/versions/:versionId",
+  json((req) => model.getVersion(req.params.versionId)),
+);
+
 // Logs for a route. ?since=<id> returns only newer rows (for polling).
 router.get(
   "/structures/:id/routes/:routeId/logs",