fix: escape quotes/newlines in YAML and gitconfig emitters

Closes #258.

`egress_render_routes` and `_render_match_entry` now pass all manifest
strings (host, auth_scheme, token_env, path/header values) through
`_yaml_str_escape` before interpolating into double-quoted YAML scalars,
preventing stray `"` or newlines from corrupting routes.yaml.

`git_gate_render_gitconfig` now calls `_gitconfig_validate_value` on
each Upstream value (and the derived alias) before writing the
`insteadOf` line, rejecting any value containing a newline that would
inject arbitrary gitconfig keys.

tests/unit/test_provision_git.py

@@ -8,6 +8,7 @@ import unittest
 
 from bot_bottle.git_gate import (
     GIT_GATE_HOSTNAME,
+    _gitconfig_validate_value,
     git_gate_render_gitconfig,
 )
 from bot_bottle.manifest import ManifestIndex
@@ -90,5 +91,42 @@ class TestGitGateGitconfigRender(unittest.TestCase):
         self.assertNotIn("gitea.dideric.is", out)
 
 
+class TestGitconfigValidateValue(unittest.TestCase):
+    """_gitconfig_validate_value rejects values that would inject gitconfig keys."""
+
+    def test_normal_url_passes(self):
+        _gitconfig_validate_value("url", "ssh://git@github.com/owner/repo.git")
+
+    def test_newline_in_url_raises(self):
+        with self.assertRaises(ValueError):
+            _gitconfig_validate_value("url", "ssh://git@github.com/owner/\nrepo.git")
+
+    def test_carriage_return_in_url_raises(self):
+        with self.assertRaises(ValueError):
+            _gitconfig_validate_value("url", "ssh://git@github.com/\rrepo.git")
+
+    def test_error_message_names_field(self):
+        with self.assertRaises(ValueError, msg="error should name the field") as ctx:
+            _gitconfig_validate_value("repos['bad'].url", "ssh://host/\npath")
+        self.assertIn("repos['bad'].url", str(ctx.exception))
+
+
+class TestGitconfigRenderRejectsNewlineInUpstream(unittest.TestCase):
+    """git_gate_render_gitconfig raises on Upstream values with newlines."""
+
+    def test_newline_in_upstream_raises(self):
+        m = ManifestIndex.from_json_obj({
+            "bottles": {"dev": {"git-gate": {"repos": {
+                "evil": {
+                    "url": "ssh://git@github.com/owner/\nfake-key = injected\nrepo.git",
+                    "key": {"provider": "static", "path": "/dev/null"},
+                },
+            }}}},
+            "agents": {"demo": {"skills": [], "prompt": "", "bottle": "dev"}},
+        })
+        with self.assertRaises(ValueError):
+            git_gate_render_gitconfig(m.bottles["dev"].git, GIT_GATE_HOSTNAME)
+
+
 if __name__ == "__main__":
     unittest.main()