refactor(egress): split DLP detector-config parsing into its own module

`egress_addon_core.py` mixed the per-route `dlp:` block parser
(`_parse_detectors` plus the detector-name and `outbound_on_match`
constants) in with the request-time scan/decision flow. Move that
config-parsing layer into a new stdlib-only `egress_dlp_config.py` as
`parse_dlp_block`, so the decision path in the core module reads
top-to-bottom without scrolling past config plumbing.

The constants and parser are re-exported from `egress_addon_core`
(and listed in `__all__`) so existing `from egress_addon_core import
ON_MATCH_*` / `OUTBOUND_DETECTOR_NAMES` callers are unchanged. The new
module ships flat into the sidecar bundle (Dockerfile.sidecars) and
uses the same flat/package import shim as its siblings. Pure refactor;
behavior and wire format unchanged.

Closes #287

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NkwFXLFff9PYPy4wgVBJp9

Dockerfile.sidecars

@@ -62,6 +62,7 @@ COPY --from=gitleaks-src /usr/bin/gitleaks /usr/bin/gitleaks
 # top-level siblings (absolute imports), matching the prior
 # Dockerfile.egress / Dockerfile.supervise layout.
 COPY bot_bottle/egress_addon_core.py /app/egress_addon_core.py
+COPY bot_bottle/egress_dlp_config.py /app/egress_dlp_config.py
 COPY bot_bottle/egress_addon.py      /app/egress_addon.py
 COPY bot_bottle/dlp_detectors.py     /app/dlp_detectors.py
 COPY bot_bottle/yaml_subset.py       /app/yaml_subset.py