config: drop dead fields from Config (memory section)

Four Config fields had no external readers, left over from earlier
features that got refactored away:

- journal_days, journal_max — journal rotation knobs that nothing
  actually consults
- prompts_dir — the old per-prompt-file directory, obsolete since
  prompt_file metadata itself went away in a prior cleanup
- api_reasoning — a reasoning-mode string that used to flow into the
  API request, superseded by per-agent reasoning_effort on AgentState

All four were only ever assigned to and never read. Drop them from the
struct, Default impl, and (as appropriate) deserialization defaults.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/config.rs

@@ -58,12 +58,8 @@ pub struct Config {
     /// Nodes loaded into subconscious agent context
     #[serde(default)]
     pub agent_nodes: Vec<String>,
-    pub journal_days: u32,
-    pub journal_max: usize,
     pub llm_concurrency: usize,
     pub agent_budget: usize,
-    #[serde(deserialize_with = "deserialize_path")]
-    pub prompts_dir: PathBuf,
     /// Resolved from agent_model → models → backend (not in config directly)
     #[serde(skip)]
     pub api_base_url: Option<String>,
@@ -85,7 +81,6 @@ pub struct Config {
     /// Number of assistant responses to score per memory. Default: 50.
     #[serde(default = "default_scoring_response_window")]
     pub scoring_response_window: usize,
-    pub api_reasoning: String,
     pub agent_types: Vec<String>,
     #[serde(default)]
     pub mcp_servers: Vec<McpServerConfig>,
@@ -118,11 +113,8 @@ impl Default for Config {
             protected_nodes: Vec::new(),
             personality_nodes: vec!["identity".into(), "core-practices".into()],
             agent_nodes: vec!["identity".into(), "core-practices".into()],
-            journal_days: 7,
-            journal_max: 20,
             llm_concurrency: 1,
             agent_budget: 1000,
-            prompts_dir: home.join(".consciousness/prompts"),
             api_base_url: None,
             api_key: None,
             api_model: None,
@@ -131,7 +123,6 @@ impl Default for Config {
             scoring_interval_secs: default_scoring_interval_secs(),
             scoring_response_window: default_scoring_response_window(),
             agent_model: None,
-            api_reasoning: "high".to_string(),
             agent_types: vec![
                 "linker".into(), "organize".into(), "distill".into(),
                 "separator".into(), "split".into(),