R2: redesign session store with wire-format identity

Replace the single-session SessionManager seam with a directory-backed
catalog. session_manager.zig -> file_system_jsonl_store.zig; the old
machinery becomes internal SessionFile, and a new FileSystemJSONLStore
implements the redesigned SessionStore vtable (create/list/resolve/latest/
load/appendMessages) minting Session/SessionInfo handles.

Session logs now record wire-format provider identity
({api_style, base_url, model, reasoning}) instead of a single provider
string or CLI aliases; no api_key material is ever stored. Disk* content
types renamed Stored*; the rich audit-oriented write record is
PersistentMessage (in-memory Message + WireIdentity + provenance).

Agent.init now takes a Session; persist_provider/persist_model display
strings deleted (banner stays alias-based, resume picks default model).
Message.metadata round-trips. Dangling-prompt recovery dropped. CLI
migrated to the catalog store for run/resume/list. Clean break, no version
bump (old logs wiped).

1 files changed, 31 insertions, 0 deletions

diff --git a/libpanto/src/config.zig b/libpanto/src/config.zig
index 8efa810..203a091 100644
--- a/libpanto/src/config.zig
+++ b/libpanto/src/config.zig
@@ -67,6 +67,37 @@ pub const ProviderConfig = union(APIStyle) {
     pub fn style(self: ProviderConfig) APIStyle {
         return @as(APIStyle, self);
     }
+
+    /// The wire-format provider identity for this config: the ground-truth
+    /// `{api_style, base_url, model, reasoning}` that a turn is sent with.
+    /// Anthropic has no reasoning-effort knob, so it reports `.default`.
+    /// Borrowed slices; valid as long as the config is.
+    pub fn wireIdentity(self: ProviderConfig) WireIdentity {
+        return switch (self) {
+            .openai_chat => |c| .{
+                .api_style = .openai_chat,
+                .base_url = c.base_url,
+                .model = c.model,
+                .reasoning = c.reasoning,
+            },
+            .anthropic_messages => |c| .{
+                .api_style = .anthropic_messages,
+                .base_url = c.base_url,
+                .model = c.model,
+                .reasoning = .default,
+            },
+        };
+    }
+};
+
+/// Wire-format provider identity (see `ProviderConfig.wireIdentity`). This
+/// is the same shape as `session_store.WireIdentity`; defined here to avoid
+/// a module cycle (config must not import session_store).
+pub const WireIdentity = struct {
+    api_style: APIStyle,
+    base_url: []const u8,
+    model: []const u8,
+    reasoning: ReasoningEffort = .default,
 };
 
 /// Compaction settings the agent consults when summarizing old history.