refactor: fail loud, tighten types, remove speculative abstraction

Fail loud at the boundary:
- substitute_template raises ConfigError on unresolved {{...}}; no more
  silent literal placeholders in download URLs.
- parse_profile raises ConfigError when 'os' is missing -- no
  raw.get("os", "linux") default that silently masks typos.
- urllib download failures wrapped to FlowError.
- bootstrap _execute_action dispatches phases explicitly and raises
  on unhandled phase; no more "anything else runs as shell".

Direct access over defensive wrapping:
- plan_bootstrap requires env; plan_install requires pm. Drop the
  dead `or os.environ` / `or detect_package_manager()` fallbacks.
- InstalledState.from_dict raises ConfigError on missing fields
  rather than .get(..., default).
- Replace `x or {}` chains with explicit `x if x is not None else {}`
  in package resolution; catalog validates type/platform-map/install
  shapes at parse.

One canonical form / direct access:
- Path.home() replaced with paths.HOME in services/packages.py and
  commands/completion.py. paths.HOME is the single source now.
- Use Path.is_relative_to for install-path containment instead of
  str.startswith.

Domain purity:
- domain/containers/resolution.resolve_mounts takes a filesystem_check
  predicate; service passes the probe in. Domain no longer touches
  the filesystem directly.

No speculative abstraction:
- Drop the `allow_sudo` field entirely. The _script_uses_sudo check
  it gated was bypassable (substring match) and gave false confidence;
  the manifest is fully user-trusted anyway.
- Delete dead terminfo_fix_command + RemoteService.fix_terminfo
  (no command surface exposes them).
- FileSystem.remove_tree no longer swallows errors via ignore_errors;
  callers opt into missing_ok if needed.

Typed enums:
- PackageDef.type, AppConfig.container_runtime as Literal[...].
  container_runtime values validated at config parse.

Completion bypasses runtime no longer:
- complete(ctx, ...) threads context; ContainerRuntime and state-file
  reads go through ctx.runtime instead of constructing primitives.

Tests added for: template raise, missing os raise, env/pm required,
unknown phase raise, no allow_sudo gate, URL download failure, install
path escape, corrupt installed.json, container_runtime Literal,
filesystem_check controls mounts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_template.py

@@ -2,6 +2,9 @@
 
 import os
 
+import pytest
+
+from flow.core.errors import ConfigError
 from flow.core.template import substitute, substitute_template
 
 
@@ -36,8 +39,18 @@ class TestSubstituteTemplate:
         ctx = {"platform": {"arch": "arm64"}}
         assert substitute_template("{{ platform.arch }}", ctx) == "arm64"
 
-    def test_preserves_unknown_templates(self):
-        assert substitute_template("{{ unknown }}", {}) == "{{ unknown }}"
+    def test_unknown_variable_raises(self):
+        with pytest.raises(ConfigError, match=r"\{\{ unknown \}\}"):
+            substitute_template("{{ unknown }}", {})
+
+    def test_nested_unresolved_raises(self):
+        with pytest.raises(ConfigError, match=r"\{\{ platform.missing \}\}"):
+            substitute_template("{{ platform.missing }}", {"platform": {"arch": "arm64"}})
+
+    def test_unresolved_env_raises(self, monkeypatch):
+        monkeypatch.delenv("SOME_NEVER_SET_VAR", raising=False)
+        with pytest.raises(ConfigError):
+            substitute_template("{{ env.SOME_NEVER_SET_VAR }}", {"env": {}})
 
     def test_non_string_passthrough(self):
         assert substitute_template(42, {}) == 42