harness-base: cargo --message-format short by default (#777)

closes #777. saves tokens by collapsing per-crate progress lines into
warning/error summaries when claude (or the operator) runs cargo
inside an agent container.

implementation: /etc/hyperhive/bash-cargo-short.sh defines a 'cargo'
bash function that injects '--message-format short' on compile
subcommands (build/check/clippy/test/run/doc/bench/install/rustc/fix).
loaded via BASH_ENV in non-interactive shells (claude's Bash tool
runs 'bash -c') and via programs.bash.interactiveShellInit in
interactive shells (operator SSH inside the container).

handles the '+toolchain' selector (cargo +nightly build), skips
injection when the caller already passes --message-format (any
form), leaves third-party cargo-* subcommands alone.

new option: hyperhive.cargo.shortMessages (default true) — agents
that parse cargo json output should set false.

nix/templates/harness-base.nix

@@ -537,6 +537,40 @@ in
     '';
   };
 
+  options.hyperhive.cargo.shortMessages = lib.mkOption {
+    type = lib.types.bool;
+    default = true;
+    example = false;
+    description = ''
+      Auto-inject `--message-format short` on cargo compile
+      subcommands (`build`, `check`, `clippy`, `test`, `run`,
+      `doc`, `bench`, `install`, `rustc`, `fix`) when claude (or
+      anything else) invokes `cargo` inside this container.
+      Saves tokens + context — the verbose default output floods
+      the response window with per-crate progress lines that
+      carry no signal beyond the warning/error summary (#777).
+
+      Implementation: a `cargo` shell function defined in
+      `/etc/hyperhive/bash-cargo-short.sh`. Loaded via `BASH_ENV`
+      for non-interactive shells (`bash -c` — what the claude
+      `Bash` tool runs) and sourced from `programs.bash.interactiveShellInit`
+      for interactive shells (operator pokes around inside the
+      container). The function:
+
+      - handles the `+toolchain` selector prefix (`cargo +nightly
+        build` works);
+      - passes through cleanly when the caller already specified
+        `--message-format` (any form);
+      - leaves non-compile subcommands (`new`, `add`, `search`,
+        third-party `cargo-*` subcommands) untouched so they
+        don't error on the unknown flag.
+
+      Set to `false` for agents that need full cargo output (e.g.
+      tooling that parses `--message-format json` programmatically
+      and doesn't pass the flag explicitly).
+    '';
+  };
+
   options.hyperhive.autoCompact = lib.mkOption {
     type = lib.types.bool;
     default = true;
@@ -712,6 +746,51 @@ in
       source = config.hyperhive.icon;
     };
 
+    # Cargo `--message-format short` injector (#777). Sourced from
+    # BASH_ENV in non-interactive shells AND interactiveShellInit
+    # so both claude's `Bash` tool and operator SSH sessions get
+    # the same compact compile output. `command cargo …` falls back
+    # to the un-wrapped binary in PATH (the rust toolchain's cargo
+    # — either from `environment.systemPackages` or from whatever
+    # `nix develop` shell the agent's working in).
+    environment.etc."hyperhive/bash-cargo-short.sh" =
+      lib.mkIf config.hyperhive.cargo.shortMessages
+        {
+          text = ''
+            # Auto-injects --message-format short on cargo compile
+            # subcommands so per-crate progress lines don't flood
+            # claude's context (#777). Bypassed when the caller
+            # already passes --message-format (any form).
+            cargo() {
+              # Strip leading +toolchain selectors (cargo +nightly …).
+              local pre=()
+              while [ "''${1:0:1}" = "+" ] && [ -n "''${1:-}" ]; do
+                pre+=("$1")
+                shift
+              done
+              case "''${1:-}" in
+                build|check|clippy|test|run|doc|bench|install|rustc|fix)
+                  local sub="$1"
+                  shift
+                  local arg
+                  for arg in "$@"; do
+                    case "$arg" in
+                      --message-format|--message-format=*)
+                        command cargo "''${pre[@]}" "$sub" "$@"
+                        return $?
+                        ;;
+                    esac
+                  done
+                  command cargo "''${pre[@]}" "$sub" --message-format short "$@"
+                  ;;
+                *)
+                  command cargo "''${pre[@]}" "$@"
+                  ;;
+              esac
+            }
+          '';
+        };
+
     environment.etc."hyperhive/bash-allow.json".text =
       builtins.toJSON config.hyperhive.allowedBashPatterns;
 
@@ -773,8 +852,27 @@ in
     }
     // lib.optionalAttrs (config.hyperhive.forge.skipNotifyReasons != [ ]) {
       HIVE_FORGE_NOTIFY_SKIP_REASONS = lib.concatStringsSep "," config.hyperhive.forge.skipNotifyReasons;
+    }
+    // lib.optionalAttrs config.hyperhive.cargo.shortMessages {
+      # Non-interactive bash invocations (claude's `Bash` tool runs
+      # `bash -c`) source $BASH_ENV at startup — drops the cargo
+      # function defined in the file above into scope without
+      # touching /etc/profile (login-only). Interactive shells
+      # source the same file via the interactiveShellInit hook
+      # below so behaviour matches across both modes (#777).
+      BASH_ENV = "/etc/hyperhive/bash-cargo-short.sh";
     };
 
+    # Interactive shells don't honour BASH_ENV — wire the same file
+    # in via the bashrc hook so operator SSH sessions get the same
+    # short-format cargo output as claude's non-interactive calls.
+    programs.bash.interactiveShellInit =
+      lib.mkIf config.hyperhive.cargo.shortMessages ''
+        if [ -r /etc/hyperhive/bash-cargo-short.sh ]; then
+          . /etc/hyperhive/bash-cargo-short.sh
+        fi
+      '';
+
     boot.isNspawnContainer = true;
 
     # Every agent gets flakes + the modern `nix` CLI out of the box.