feat(features): per-feature freshness spec — JIT vs batched (#61)

Each ml/features/*.py now declares freshness, source, and fallback per
feature. ProfileFeature gains ttl_sec (mirrored from registry.ts),
freshness="batched", source, and fallback. context.py adds
ContextFeatureSpec + CONTEXT_FEATURES for the three JIT features
(hour_of_day, day_of_week, tasks). CI test parses ttlSec from registry.ts
to catch drift. ml/README updated with split JIT/batched feature contract.

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

ml/features/context.py

@@ -2,12 +2,56 @@
 Context assembler — converts raw user signals into a PromptContext for LLM tip generation.
 
 Usage:
-    from ml.features.context import build_context
+    from ml.features.context import build_context, CONTEXT_FEATURES
     ctx = build_context(tasks, hour_of_day=9, day_of_week=2)
+
+Feature-spec (issue #61):
+  All context features are JIT — they are assembled at request time from live
+  sources (system clock, caller-supplied task list) rather than read from a
+  cached profile store. They carry no TTL because they are never persisted.
 """
 
 from __future__ import annotations
 from dataclasses import dataclass, field
+from typing import Literal
+
+
+@dataclass(frozen=True)
+class ContextFeatureSpec:
+    name: str
+    dtype: Literal["numeric", "categorical", "list"]
+    freshness: Literal["jit", "batched"]
+    source: str
+    fallback: str
+    description: str
+
+
+CONTEXT_FEATURES: tuple[ContextFeatureSpec, ...] = (
+    ContextFeatureSpec(
+        name="hour_of_day",
+        dtype="numeric",
+        freshness="jit",
+        source="request",
+        fallback="12",
+        description="Current hour (0–23), supplied by the caller at score time.",
+    ),
+    ContextFeatureSpec(
+        name="day_of_week",
+        dtype="numeric",
+        freshness="jit",
+        source="request",
+        fallback="0",
+        description="ISO weekday (0=Monday … 6=Sunday), supplied by the caller at score time.",
+    ),
+    ContextFeatureSpec(
+        name="tasks",
+        dtype="list",
+        freshness="jit",
+        source="todoist-integration",
+        fallback="[]",
+        description="User's open tasks fetched live from the Todoist integration at request time.",
+    ),
+)
 
 
 @dataclass