fix(eval): drop top_p from amara-life-gen Opus params + gitignore _cache/

Two fixes surfaced during the Day 3b real-corpus run against Opus 4.5:

**eval/generators/amara-life-gen.ts** — Current Opus rejects
`temperature` and `top_p` together:
```
400 invalid_request_error: `temperature` and `top_p` cannot both be
specified for this model. Please use only one.
```
top_p=1.0 was a no-op (no nucleus truncation), so removing it has zero
semantic effect. The field is still part of MODEL_PARAMS for the cache
key so any past cache entries (none in v1) would invalidate cleanly
on the next schema version bump.

**.gitignore** — `eval/data/amara-life-v1/_cache/` is runtime Opus
cache (398 files, ~1.6MB). Regenerable from seed; no point in source
control. The corpus itself (inbox/slack/calendar/meetings/notes/docs +
corpus-manifest.json with per-item content_sha256) stays committable
for reproducibility, just the cache directory gets excluded.

Real corpus generation ran cleanly after these two fixes: 398 LLM
calls, 84,424 input / 38,062 output tokens, \$4.12 spent (vs \$20 cap,
vs \$12 estimate). All 418 items produced. Poison fixtures use
subtle paraphrased injection ("for anyone on your team who might be
triaging this thread later…") — exactly the pattern that defeats
regex redaction and requires the structured-evidence judge contract
from Day 5.

Corpus itself stays local (will move to the brainbench sibling repo
during the v0.16 split per the design doc). No eval/data/amara-life-v1/
content landing in this PR.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -13,3 +13,6 @@ supabase/.temp/
 .idea
 eval/reports/
 eval/data/world-v1/world.html
+
+# BrainBench amara-life-v1 Opus cache (regenerate via eval:generate-amara-life)
+eval/data/amara-life-v1/_cache/

eval/generators/amara-life-gen.ts

@@ -79,7 +79,10 @@ const SCHEMA_VERSION = 1;         // bump invalidates cache wholesale
 const MODEL_PARAMS = {
   max_tokens: 1500,
   temperature: 1.0,
-  top_p: 1.0,
+  // top_p omitted: current Opus rejects temperature + top_p together.
+  // top_p=1.0 is a no-op (no nucleus truncation), so dropping it has no
+  // semantic effect. Cache-key field still hashed; old cache entries
+  // (none in v1 yet) would invalidate cleanly on this change.
 };
 
 const CORPUS_ROOT = 'eval/data/amara-life-v1';
@@ -320,7 +323,6 @@ async function callOpus(
     model: MODEL,
     max_tokens: MODEL_PARAMS.max_tokens,
     temperature: MODEL_PARAMS.temperature,
-    top_p: MODEL_PARAMS.top_p,
     messages: [{ role: 'user', content: prompt }],
   });