feat: add long-term memory systems with fused retrieval and memory explorer

Phase 5 (ADR-0003). pkg/concepts fixes the epistemic vocabulary
(observation, fact, memory, claim, hypothesis, belief, prediction,
assumption, inference, opinion), memory kinds and relation names.

internal/memory stores episodic, semantic, procedural and
autobiographical memories in one mind-scoped table with trust-derived
credibility, importance, salience and decaying activation. Retrieval
fuses full-text rank (any-term by default), optional pgvector similarity,
recency, activation, importance and credibility, and returns the
component scores so every recall can be explained. Decay lowers
activation and never deletes; retraction and supersession keep rows for
provenance. Relationships are constrained to one mind at insert time.
Embeddings live in a conditional pgvector table keyed by model so vector
spaces are never mixed; without an embedder the system degrades to text
and graph retrieval.

Attention gates encoding: the attend handler turns admitted observations
into episodic memories (operator changes become autobiographical ones)
with memory_created events; the retrieve process proposes recalled
memories as candidates and admitted recalls are reinforced with
memory_retrieved events carrying their scoring components.

Memory explorer UI (search with component breakdown, timeline, filters,
operator knowledge entry), memory detail with provenance, relationships,
citing events and an inaccurate-memory correction; API; noemactl memory
commands; decay and embed-missing background jobs.

web/templates/pages/mind.html

@@ -24,7 +24,7 @@
     {{$st := .Data.Stats}}
     <div class="stat-value small">{{$st.Total}} <span class="muted small">events in 24h</span></div>
     <div class="cats small">{{range $k, $v := $st.ByCategory}}<span class="etype cat-{{$k}}">{{$k}} {{$v}}</span> {{end}}</div>
-    <p class="small"><a href="/minds/{{$m.Slug}}/thoughts">Open thought stream</a></p>
+    <p class="small"><a href="/minds/{{$m.Slug}}/thoughts">Open thought stream</a> · <a href="/minds/{{$m.Slug}}/memory">{{.Data.MemStats.Total}} memories</a>{{range $k, $v := .Data.MemStats.ByKind}} <span class="badge k-{{$k}}">{{$k}} {{$v}}</span>{{end}}</p>
     {{if .Principal.User.Role.AtLeast "operator"}}
     <form method="post" action="/minds/{{$m.Slug}}/observe" class="stack" style="margin-top:.75rem">
       <input type="hidden" name="_csrf" value="{{.CSRF}}">