feat(#2023): inject OTEL auth header at runtime, never in the nix store (mara: b)

Per mara: a secret in the nix store is not acceptable. The non-secret
OTEL config (telemetry-enable, endpoint, protocol, resource attributes)
stays in the world-readable managed settings json; the auth header is
handled separately at runtime so it never touches the store.

New hive-otel-header oneshot (only when otel.enable && headersCredential
is set): inherits the forwarded otel-headers systemd credential via
LoadCredential, reads it at start, and merges OTEL_EXPORTER_OTLP_HEADERS
into the agent's 0600 ~/.claude/settings.json env block via jq. claude
layers the user env on top of the managed settings, so both the harness
turn-loop and hivectl choom (same agent user) export with auth. The
token is read from disk at start and never copied into the nix store or
the world-readable managed file.

Ordering is best-effort (before=, not a hard dep): a failure leaves the
harness running and telemetry exporting unauthenticated. headersCredential
option description updated to reflect it's now wired.

nix fmt clean.

nix/templates/harness-base.nix

@@ -30,9 +30,13 @@ let
   # Shipped via the managed claude settings json (below), which claude
   # auto-discovers for BOTH the harness turn-loop and `hivectl choom` —
   # so telemetry parity is declarative, with no launch wrapper. The
-  # auth header (`otel.headersCredential`) is deliberately NOT included:
-  # it's a secret and the settings file is world-readable; authenticated
-  # collectors need a runtime mechanism (tracked as a follow-up).
+  # auth header (`otel.headersCredential`) is deliberately NOT included
+  # here: it's a secret and this file lives in the world-readable nix
+  # store. It's injected at *runtime* into the agent's `0600`
+  # `~/.claude/settings.json` by the `hive-otel-header` oneshot below
+  # (claude merges the `env` from the user settings on top of these
+  # managed ones), so the token is read from disk at start and never
+  # touches the store.
   otelSettingsEnv = {
     CLAUDE_CODE_ENABLE_TELEMETRY = "1";
     OTEL_METRICS_EXPORTER = "otlp";
@@ -264,18 +268,18 @@ in
       internal = true;
       description = ''
         Absolute path to an operator-provided secret file whose contents
-        would become `OTEL_EXPORTER_OTLP_HEADERS` (e.g.
+        become `OTEL_EXPORTER_OTLP_HEADERS` (e.g.
         `Authorization=Bearer <token>`). Host-driven via
         `services.hyperhive.otel.headersCredential`.
 
-        **Not yet wired up.** OTEL config now ships through the managed
-        claude settings json (`/etc/claude-code/managed-settings.json`),
-        which is world-readable, so a secret auth header can't be baked
-        into it. Setting this option currently has no effect — the
-        unauthenticated export path is the only one implemented. A
-        follow-up will inject the header at runtime (e.g. the harness
-        writing it into the agent's `0600` `~/.claude/settings.json`),
-        keeping it out of the nix store and the world-readable file.
+        The rest of the OTEL config ships in the world-readable managed
+        claude settings json, but the header is a secret, so it's handled
+        separately: hive-c0re forwards this file into the container's
+        systemd credential store, and the `hive-otel-header` oneshot
+        reads it at runtime (`LoadCredential`) and writes it into the
+        agent's `0600` `~/.claude/settings.json` `env` block. The token
+        is read from disk at start and never copied into the nix store or
+        the world-readable settings file.
       '';
     };
 
@@ -1133,6 +1137,49 @@ in
             '. + { env: $env }' ${baseSettings} > "$out"
         '';
 
+    # Inject the OTEL auth header (a secret) into the agent's *user*
+    # claude settings at runtime, keeping it out of the world-readable
+    # managed settings json above and out of the nix store entirely.
+    # hive-c0re forwards the operator's `headersCredential` file into
+    # this container's systemd credential store; this oneshot reads it
+    # via `LoadCredential` at start and merges `OTEL_EXPORTER_OTLP_HEADERS`
+    # into `~/.claude/settings.json` (0600, agent-owned). claude layers
+    # the user `env` on top of the managed one, so both the harness
+    # turn-loop and `hivectl choom` (same agent user) pick it up. Ordering
+    # is best-effort (`before`, not a hard dep): if it fails the harness
+    # still starts and telemetry just exports unauthenticated.
+    systemd.services.hive-otel-header =
+      lib.mkIf (config.hyperhive.otel.enable && config.hyperhive.otel.headersCredential != null)
+        {
+          description = "Inject the OTEL auth header into the agent's claude user settings";
+          wantedBy = [ "multi-user.target" ];
+          before = [ "hive-ag3nt.service" ];
+          serviceConfig = {
+            Type = "oneshot";
+            RemainAfterExit = true;
+            User = userName;
+            Group = userName;
+            LoadCredential = [ "otel-headers" ];
+            ExecStart = pkgs.writeShellScript "hive-otel-header" ''
+              set -eu
+              umask 077
+              hdr="$CREDENTIALS_DIRECTORY/otel-headers"
+              [ -r "$hdr" ] || exit 0
+              dir=${homeDir}/.claude
+              settings="$dir/settings.json"
+              mkdir -p "$dir"
+              base='{}'
+              [ -s "$settings" ] && base="$(cat "$settings")"
+              printf '%s' "$base" | ${pkgs.jq}/bin/jq \
+                --rawfile h "$hdr" \
+                '.env = ((.env // {}) + { OTEL_EXPORTER_OTLP_HEADERS: ($h | rtrimstr("\n")) })' \
+                > "$settings.tmp"
+              mv "$settings.tmp" "$settings"
+              chmod 0600 "$settings"
+            '';
+          };
+        };
+
     # Merged frontend static tree. Base = `${frontend.dist}/agent/`,
     # then each `extraFiles` entry is laid on top at its `target`
     # path. The runCommand derivation aborts on overwrite so a