chore(.gitignore): exclude AI-agent scratch dirs and stray fixtures

The repo root has been accumulating AI-coding-agent dropouts that have
no project contract value:

  .codex/, .codex-app-schema/, .codex-app-ts/   — OpenAI Codex CLI
  AGENTS.md, GEMINI.md                          — per-agent instructions
  CLAUDE.md                                     — same shape
  .github/copilot-instructions.md               — GitHub Copilot hints

These are operator-side preferences. If something needs to be canonical
for the project, it goes in docs/ explicitly.

Also adding backend/tests/test_carrier_tracker_region_centers.py —
a stale fixture that referenced fields (region, source_detail,
position_label, position_source_type, position_confidence='low')
that don't exist in the current `_parse_carrier_positions_from_news`
implementation. The real coverage for that function lives in
tests/test_carrier_tracker_quality.py from PR #285.

.gitignore

@@ -261,3 +261,32 @@ backend/data/wormhole_stdout.log
 
 # Compressed snapshot archives (can be 100 MB+)
 *.json.gz
+
+# ──────────────────────────────────────────────────────────────────────
+# AI assistant / coding-agent scratch
+# ──────────────────────────────────────────────────────────────────────
+# Per-tool config + scratch directories. These are private to whichever
+# coding agent the operator happens to be using and have no business in
+# the repo. If a tool's instructions need to be canonical for the project,
+# we'll put them in docs/ explicitly — not let the agent dump them at the
+# repo root.
+
+# OpenAI Codex CLI
+.codex/
+.codex-app-schema/
+.codex-app-ts/
+
+# Per-agent instruction files dropped at repo root by various tools.
+# These are operator-side preferences, not part of the project contract.
+AGENTS.md
+GEMINI.md
+CLAUDE.md
+.github/copilot-instructions.md
+
+# Stale AI-generated test file that referenced fields that don't exist in
+# the current `_parse_carrier_positions_from_news` implementation. Kept
+# ignored so it doesn't accidentally get committed if it shows up again
+# from a tool that's working off an out-of-date understanding of the
+# module. If a real test for that function is needed, write it under a
+# meaningful name in tests/test_carrier_tracker_quality.py.
+backend/tests/test_carrier_tracker_region_centers.py