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>

migrations/004_add_versions.sql

@@ -0,0 +1,15 @@
+-- Append-only version history for the code-bearing entities: route handlers,
+-- template content, and db libraries. Every content-changing save records a
+-- snapshot here (see recordVersion in db.js). The live rows in routes/templates/
+-- dbs remain the current version; this table is the "plumber's" undo trail.
+CREATE TABLE versions (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    entity_type TEXT NOT NULL CHECK(entity_type IN ('route', 'template', 'db')),
+    entity_id INTEGER NOT NULL,
+    structure_id INTEGER,
+    snapshot TEXT NOT NULL,           -- JSON of the versioned fields
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX idx_versions_entity ON versions (entity_type, entity_id, id);
+CREATE INDEX idx_versions_structure ON versions (structure_id, id);