Make the system portable: split config from profile, remove hard-coding

Audit found three things compiled in that shouldn't have been: the User-Agent
carried the author's domain, the cluster threshold and window were module
constants the pipeline never overrode, and the embedding model was pinned --
which blocked the one change most likely to be wanted, swapping in a
multilingual model for native-language sources.

Configuration is now three files with distinct roles: config.toml (machinery),
profile.toml (who the reader is), sources.toml (what to read). All are
relocatable via NEWS_CONFIG_DIR. news/*.py now contains no interest, place name,
feed URL, or personal identifier.

tests/test_config.py keeps it that way: it builds a configuration for an
invented reader and asserts none of the author's identity or preferences reaches
the running code. Writing it immediately found a real bug -- CONFIG_DIR was
resolved at import time, so NEWS_CONFIG_DIR silently did nothing if set after
the module was first imported. Now resolved per instantiation.

Documented the coupling that portability creates: [clustering].threshold and
[scoring].veto_absolute are cosine constants calibrated against one corpus with
one model, so changing [embedding].model invalidates both.

profile/config.toml

@@ -0,0 +1,53 @@
+# Runtime configuration and tuning knobs.
+#
+# Three-file split, so a new user knows what to touch:
+#   config.toml   this file -- machinery. Sane defaults; rarely edited.
+#   profile.toml  who you are: interests, vetoes, importance, delivery.
+#   sources.toml  what to read.
+#
+# Nothing here is specific to any person. Everything personal lives in the
+# other two files.
+
+[http]
+# Sent on every outbound request. Feed operators use it to identify traffic and
+# some will block a generic or absent agent, so put a real contact here.
+# CHANGE THIS if you are not Connor -- it is the contact address feed operators
+# see, and pointing them at someone else's domain is rude and unhelpful.
+user_agent = "news/0.1 (+https://news.rcjohnstone.com; personal feed reader; one user)"
+timeout_secs = 30
+connect_timeout_secs = 10
+concurrency = 8
+max_body_bytes = 10485760
+
+[embedding]
+# Any model fastembed can serve. `dim` must match the model.
+#
+# The default is English-only. For native-language sources use a multilingual
+# model -- "intfloat/multilingual-e5-small" (384 dim) is the usual choice.
+# Be aware that changing the model INVALIDATES the calibrated constants below
+# and in [scoring]: they are cosine thresholds, and a different model puts
+# similarity on a different scale. Re-run the calibration before trusting them.
+model = "BAAI/bge-small-en-v1.5"
+dim = 384
+
+[clustering]
+# Calibrated by inspection on a 3,885-item corpus; see CALIBRATION.md.
+# 0.70 merged "Solheim Cup capsules" with "FIBA Women's World Cup Glance";
+# 0.86 under-merged. Over-merging is the worse failure because it inflates the
+# corroboration signal that feeds importance.
+#
+# Re-tune if your sources differ substantially in style or language.
+threshold = 0.82
+window_hours = 48
+
+[scoring]
+# Compare blocks by z-score WITHIN each block, never by raw cosine: cosine
+# magnitude is not comparable across blocks, and a plain max let one block win
+# 78% of all items. See CALIBRATION.md.
+normalize = "zscore-per-block"
+
+# Veto is not a plain similarity threshold. A story can be genuinely adjacent to
+# a vetoed topic and still be wanted, so an item is vetoed on unambiguous
+# similarity OR when the veto signal beats the best interest signal by a margin.
+veto_absolute = 0.60
+veto_margin_over_interest = 0.3