ver 1.5.1 — duplicate-gate precision (common tokens, cross-kind)

Live 1.5.0 use caught the gate blocking a decision note over the single
vault-common token "echo" (min-normalized overlap gives any one-token
entity a 1.0 score). New echo_index.gate_candidates() + token_df():

- gate blocks same-kind candidates only; cross-kind name collisions
  (a decision titled after its project) warn instead of blocking
- a lone shared token blocks only when unique to that entity (df == 1);
  multi-token overlaps still block

fuzzy_candidates (advisory warnings) and exit-76/--merge-into/--force
semantics unchanged. +3 offline unit tests, gate e2e cases restructured
+2 new; verified live both directions. All suites green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

echo-memory.plugin.src/.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "echo-memory",
-  "version": "1.5.0",
+  "version": "1.5.1",
   "description": "Persistent memory via the ECHO Obsidian vault over the Obsidian Local REST API. Cross-platform Python client: one-call capture/resolve/recall/link/triage over an entity index, hybrid BM25 + graph recall spanning entities + sessions/journal (recency/status-aware), a pre-write duplicate gate, complete-frontmatter capture, session hooks that self-fire load/reflect, offline write-ahead queue, lock-guarded concurrency, linter-enforced routing, and /echo-* commands.",
   "author": {
     "name": "Jason Stedwell"