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.

tests/unit/test_yaml_subset.py

@@ -5,7 +5,7 @@ actually use, and every rejection case the PRD enumerates."""
 import textwrap
 import unittest
 
-from claude_bottle.log import Die
+from claude_bottle.yaml_subset import YamlSubsetError
 from claude_bottle.yaml_subset import parse_frontmatter, parse_yaml_subset
 
 
@@ -60,7 +60,7 @@ class TestForbiddenBoolLikes(unittest.TestCase):
     """Ambiguous bool-ish tokens have to be quoted explicitly."""
 
     def _expect_die(self, src: str):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             _y(src)
 
     def test_yes_dies(self):
@@ -81,7 +81,7 @@ class TestForbiddenBoolLikes(unittest.TestCase):
 
 class TestForbiddenScalarShapes(unittest.TestCase):
     def _expect_die(self, src: str):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             _y(src)
 
     def test_bare_date_dies(self):
@@ -120,14 +120,14 @@ class TestMapping(unittest.TestCase):
         self.assertEqual({"outer": {"inner": "hello", "other": 5}}, out)
 
     def test_duplicate_key_dies(self):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             _y("""
                 a: 1
                 a: 2
             """)
 
     def test_key_must_be_bare_identifier(self):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             _y('"weird key": 1\n')
 
 
@@ -202,20 +202,20 @@ class TestInline(unittest.TestCase):
         )
 
     def test_nested_flow_dies(self):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             _y("l: [[1, 2], [3, 4]]\n")
 
 
 class TestForbiddenConstructs(unittest.TestCase):
     def test_anchor_dies(self):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             _y("""
                 a: &anchor 1
                 b: *anchor
             """)
 
     def test_multiline_block_scalar_dies(self):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             _y("""
                 k: |
                   line 1
@@ -223,11 +223,11 @@ class TestForbiddenConstructs(unittest.TestCase):
             """)
 
     def test_tag_dies(self):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             _y("k: !!str hello\n")
 
     def test_tab_in_indent_dies(self):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             _y("a:\n\tb: 1\n")
 
 
@@ -306,7 +306,7 @@ class TestFrontmatter(unittest.TestCase):
         self.assertEqual(text, body)
 
     def test_unclosed_frontmatter_dies(self):
-        with self.assertRaises(Die):
+        with self.assertRaises(YamlSubsetError):
             parse_frontmatter("---\nbottle: dev\nno closing")
 
     def test_body_preserves_blank_lines(self):