refactor(egress): write routes.yaml as actual YAML, not JSON-in-yml

`egress_render_routes` now emits hand-rolled YAML in the same style
as `pipelock_render_yaml`. The egress addon parses it via
`yaml_subset.parse_yaml_subset` — the same parser the manifest
loader + pipelock_apply use.

Why bother: routes.yaml is bind-mounted into the egress sidecar
AND surfaced to operators through `routes edit` (PRD 0019). JSON-
in-yml renders ugly in $EDITOR and signals "this is data" rather
than "this is config you can read at a glance". Real YAML reads
cleanly.

Mechanics:

  - `yaml_subset.py` drops its `claude_bottle.log` dependency.
    Errors now raise `YamlSubsetError` (a `ValueError`); the
    manifest loader + pipelock_apply catch it at the boundary
    and forward to `die` / `PipelockApplyError` so callers see
    the same behavior they did before.
  - `Dockerfile.egress` adds one COPY line for `yaml_subset.py`
    so it sits flat in `/app/` next to the addon. The addon
    uses an absolute-import-with-fallback shim so the same file
    works inside the container AND from the host's unit tests.
  - `egress_apply._merge_single_route` round-trips current
    routes.yaml through `parse_yaml_subset` + a new
    `_render_routes_payload` helper instead of `json.loads` +
    `json.dumps`.

End-to-end: rebuilt the egress image, ran `./cli.py start` to a
full bring-up, confirmed the addon's boot log shows `egress:
loaded 9 route(s)` — i.e., the YAML parses inside the container.
453 unit + 3 integration tests pass.

Dockerfile.egress

@@ -18,12 +18,15 @@ FROM mitmproxy/mitmproxy:11.1.3
 
 USER root
 
-# The addon ships as two files. `_core.py` is pure-logic, importable
-# both inside the container and from the host's tests; `_addon.py` is
-# the mitmproxy hook wrapper. Both land flat in /app/ so mitmdump's
-# loader finds them as top-level sibling modules.
+# The addon ships as three files. `_core.py` is pure-logic,
+# importable both inside the container and from the host's tests;
+# `_addon.py` is the mitmproxy hook wrapper; `yaml_subset.py` is
+# the stdlib-only YAML parser the addon uses to read routes.yaml.
+# All three land flat in /app/ so mitmdump's loader resolves them
+# as top-level sibling modules (absolute imports).
 COPY claude_bottle/egress_addon_core.py /app/egress_addon_core.py
 COPY claude_bottle/egress_addon.py /app/egress_addon.py
+COPY claude_bottle/yaml_subset.py /app/yaml_subset.py
 
 # Pre-create the runtime directories the backend's start step will
 # `docker cp` into. docker cp does not create intermediate dirs, so