feat: associative cognition — thoughts arise from spreading activation, not a re-read bio

Replaces the thought loop's grist (recent-convo + her own saved narrative, the
feedback-loop attractor) with a model of how a thought actually arises:

  seed (salience-weighted: a recent moment / resurfaced memory / feed item)
   -> spreading activation: embed the seed, let it light up associatively-near
      material across ALL her stores (conversations, gists, her own journal/
      thoughts), blended by relevance + recency + noise; optional 2nd hop for leaps
   -> her self-narrative stays the LENS (supplied as interiority), not the input
   -> the thought is generated from what lit up, routed through a faculty
      (notice / connect / abstract / project / feel)
   -> journaled + embedded, so it can light up in future cycles

This breaks the feedback loop structurally: the narrative is no longer reread and
paraphrased each cycle; grist is genuinely associative and varied; and her past
thoughts re-activate (continuity without calcification).

- lyra/cognition.py (new): spontaneous_seed, activate (spreading activation),
  constellation_block, faculties.
- memory.py: journal entries now embedded; recall_journal(); backfill_journal_embeddings()
  (ran once: 341 past entries embedded so her history is associatively retrievable).
- thoughts.think(): new-thread mode now uses the associative engine; dropped _grist().
- tests: test_cognition.py (recall_journal ranking, activation, seeding) + fixture
  reloads cognition. Suite 72 green, ruff clean.

Honest scope: this fixes the mechanism (how thoughts arise). The residual
"be useful for Brian" voice drift is the separate model/fine-tune problem.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_thoughts.py

@@ -23,6 +23,8 @@ def lyra(tmp_path, monkeypatch):
     importlib.reload(self_state)
     import lyra.feeds as feeds
     importlib.reload(feeds)
+    import lyra.cognition as cognition
+    importlib.reload(cognition)
     import lyra.thoughts as thoughts
     importlib.reload(thoughts)