hive-priv: replace json! with typed structs for account sidecar files

Cargo.lock

@@ -1795,6 +1795,7 @@ dependencies = [
  "anyhow",
  "hive-priv-sock",
  "libc",
+ "serde",
  "serde_json",
  "tokio",
  "tracing",

hive-priv/Cargo.toml

@@ -11,6 +11,7 @@ workspace = true
 anyhow.workspace = true
 hive-priv-sock.workspace = true
 libc.workspace = true
+serde.workspace = true
 serde_json.workspace = true
 tokio.workspace = true
 tracing.workspace = true

hive-priv/src/main.rs

@@ -27,6 +27,7 @@ use hive_priv_sock::{
     NetworkIsolation, PAUSED_MARKER_FILE, PRIV_SOCK, PrivEvent, PrivRequest, PrivResponse,
     PrivStream, PrivStreamLine, SIBLING_CONTAINERS,
 };
+use serde::Serialize;
 use tokio::io::{AsyncWriteExt, BufReader};
 use tokio::net::unix::OwnedWriteHalf;
 use tokio::net::{UnixListener, UnixStream};
@@ -468,7 +469,10 @@ async fn exec(
             // when both `account` and `homeserver` are present; the account
             // suffix is already validated above.
             if let (Some(a), Some(hs)) = (account, homeserver) {
-                let meta = serde_json::json!({ "homeserver": hs }).to_string();
+                let meta = serde_json::to_string(&MatrixAccountSidecar {
+                    homeserver: hs.as_str(),
+                })
+                .context("serialize matrix account sidecar")?;
                 write_agent_state_file(agent_name, &format!("matrix-account-{a}.json"), &meta)?;
             }
             Ok(res)
@@ -497,7 +501,10 @@ async fn exec(
             )?;
             // Sidecar carries the base URL — there's no host-side nix config
             // for extra forges, so this is the only place it's persisted.
-            let meta = serde_json::json!({ "base_url": base_url }).to_string();
+            let meta = serde_json::to_string(&ForgeSidecar {
+                base_url: base_url.as_str(),
+            })
+            .context("serialize forge account sidecar")?;
             write_agent_state_file(agent_name, &format!("forge-{label}.json"), &meta)?;
             Ok(res)
         }
@@ -957,6 +964,32 @@ fn write_state_file_nofollow(dir: &Path, filename: &str, content: &str) -> Resul
     Ok(file)
 }
 
+/// Sidecar written alongside an extra matrix account's token
+/// (`matrix-account-<name>.json`) so `hive-matrix-mcp` can auto-discover
+/// the account's homeserver without a static `matrixAccounts` config
+/// entry. Read side: `hive-matrix-mcp/src/accounts.rs`'s
+/// `read_account_homeserver` (deliberately reads via a bare
+/// `serde_json::Value` rather than this shape — that side treats a
+/// malformed/missing sidecar as "skip this account" rather than an
+/// error, so it stays loosely typed; this side is the one place the
+/// file is written, so it gets the precise shape).
+#[derive(Serialize)]
+struct MatrixAccountSidecar<'a> {
+    homeserver: &'a str,
+}
+
+/// Sidecar written alongside a dashboard-provisioned extra forge
+/// account's token (`forge-<label>.json`) so `hive-forge` can resolve
+/// the account's base URL. Read side: `hive-forge/src/client.rs`'s own
+/// (separately defined, deserialize-only) `ForgeSidecar` — same field
+/// name (`base_url`), no shared crate between `hive-priv` and
+/// `hive-forge` to hang a common type off, so the two structs are
+/// pinned to the same JSON key by convention, not by the compiler.
+#[derive(Serialize)]
+struct ForgeSidecar<'a> {
+    base_url: &'a str,
+}
+
 /// Shared helper for `WriteAgentForgeToken` and `WriteAgentMatrixToken`.
 /// Writes `content` to `AGENT_STATE_ROOT/<agent_name>/state/<filename>`,
 /// chowns to the agent user (derived from the state dir's existing owner),