Refactor config access around an Ecto schema

#### Context

The config layer had grown around NimbleOptions and a large getter surface. This branch moves parsing to an Ecto schema and simplifies callers to read typed nested settings directly.

#### TL;DR

*Replace the bespoke config access layer with an Ecto-backed schema and typed settings access.*

#### Summary

- Replace the old config extraction path with an Ecto embedded schema and defaults
- Collapse `Config` down to `settings/0`, `settings!/0`, and a small runtime helper surface
- Update callers and tests to read nested config structs directly instead of one-off getter wrappers
- Keep the repo green under format, lint, coverage, and dialyzer by adding the missing schema specs and tests

#### Alternatives

- Keep NimbleOptions and shrink the wrapper layer incrementally, but that would preserve the custom parser shape
- Keep the strict refactor without extra schema tests, but that left `make -C elixir all` red on coverage and dialyzer

#### Test Plan

- [x] `make -C elixir all`
- [x] `cd elixir && mix test test/symphony_elixir/workspace_and_config_test.exs --warnings-as-errors`

Author: frantic-openai

elixir/lib/symphony_elixir/agent_runner.ex

@@ -47,7 +47,7 @@ defmodule SymphonyElixir.AgentRunner do
   defp send_codex_update(_recipient, _issue, _message), do: :ok
 
   defp run_codex_turns(workspace, issue, codex_update_recipient, opts) do
-    max_turns = Keyword.get(opts, :max_turns, Config.agent_max_turns())
+    max_turns = Keyword.get(opts, :max_turns, Config.settings!().agent.max_turns)
     issue_state_fetcher = Keyword.get(opts, :issue_state_fetcher, &Tracker.fetch_issue_states_by_ids/1)
 
     with {:ok, session} <- AppServer.start_session(workspace) do
@@ -136,7 +136,7 @@ defmodule SymphonyElixir.AgentRunner do
   defp active_issue_state?(state_name) when is_binary(state_name) do
     normalized_state = normalize_issue_state(state_name)
 
-    Config.linear_active_states()
+    Config.settings!().tracker.active_states
     |> Enum.any?(fn active_state -> normalize_issue_state(active_state) == normalized_state end)
   end
 

elixir/lib/symphony_elixir/codex/app_server.ex

@@ -143,7 +143,7 @@ defmodule SymphonyElixir.Codex.AppServer do
 
   defp validate_workspace_cwd(workspace) when is_binary(workspace) do
     workspace_path = Path.expand(workspace)
-    workspace_root = Path.expand(Config.workspace_root())
+    workspace_root = Path.expand(Config.settings!().workspace.root)
 
     root_prefix = workspace_root <> "/"
 
@@ -172,7 +172,7 @@ defmodule SymphonyElixir.Codex.AppServer do
             :binary,
             :exit_status,
             :stderr_to_stdout,
-            args: [~c"-lc", String.to_charlist(Config.codex_command())],
+            args: [~c"-lc", String.to_charlist(Config.settings!().codex.command)],
             cd: String.to_charlist(workspace),
             line: @port_line_bytes
           ]
@@ -277,7 +277,14 @@ defmodule SymphonyElixir.Codex.AppServer do
   end
 
   defp await_turn_completion(port, on_message, tool_executor, auto_approve_requests) do
-    receive_loop(port, on_message, Config.codex_turn_timeout_ms(), "", tool_executor, auto_approve_requests)
+    receive_loop(
+      port,
+      on_message,
+      Config.settings!().codex.turn_timeout_ms,
+      "",
+      tool_executor,
+      auto_approve_requests
+    )
   end
 
   defp receive_loop(port, on_message, timeout_ms, pending_line, tool_executor, auto_approve_requests) do
@@ -820,7 +827,7 @@ defmodule SymphonyElixir.Codex.AppServer do
   end
 
   defp await_response(port, request_id) do
-    with_timeout_response(port, request_id, Config.codex_read_timeout_ms(), "")
+    with_timeout_response(port, request_id, Config.settings!().codex.read_timeout_ms, "")
   end
 
   defp with_timeout_response(port, request_id, timeout_ms, pending_line) do