fix(fleet): export MOSAIC_AGENT_CLASS into the agent pane so personas inject

A3a/A3b ship a per-agent persona contract that the launcher composes from
process.env.MOSAIC_AGENT_CLASS (compose-contract -> readPersonaContractBlock,
launch.js). generateAgentEnv already writes MOSAIC_AGENT_CLASS to
agents/<name>.env, but start-agent-session.sh only re-exported
MOSAIC_AGENT_NAME into the pane snippet — never the class.

The pane inherits the tmux SERVER environment, not this script's env nor the
systemd unit's EnvironmentFile, so the class set in the .env was invisible
in-pane. The launcher then saw an undefined class and silently injected NO
persona contract: every fleet agent came up class-less while `fleet ps`
reported healthy. The comms block kept working only because it keys on
MOSAIC_AGENT_NAME (which IS re-exported).

Fix: re-export MOSAIC_AGENT_CLASS into PANE_SHELL_SNIPPET exactly as
MOSAIC_AGENT_NAME, %q-quoted (empty/unset class -> '' is a harmless no-op,
matching readPersonaContractBlock's empty-class contract). Verified live: a
relaunched agent now carries MOSAIC_AGENT_CLASS in its pane env and the
matching `# Persona Contract (<class>)` block in its composed prompt.

Adds a regression guard to test-start-agent-session.sh asserting the pane
snippet exports both MOSAIC_AGENT_NAME and the per-agent MOSAIC_AGENT_CLASS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/mosaic/framework/tools/fleet/start-agent-session.sh

@@ -114,10 +114,21 @@ MOSAIC_RUNTIME_BIN_PREFIX=$(_build_runtime_bin_prefix)
 # safe single bash token regardless of the name's characters.
 AGENT_NAME_Q=$(printf '%q' "$AGENT_NAME")
 
+# MOSAIC_AGENT_CLASS must ALSO be exported INTO the pane, for the same reason as
+# MOSAIC_AGENT_NAME above: the pane inherits the tmux SERVER environment (not this
+# script's env, and not the systemd unit's EnvironmentFile), so the per-agent class
+# written to agents/<name>.env would otherwise be invisible in-pane. The launcher
+# composes the persona contract from process.env.MOSAIC_AGENT_CLASS at launch
+# (compose-contract -> readPersonaContractBlock); without this export it sees an
+# undefined class and silently injects NO persona contract. %q-quote it so it is a
+# safe single bash token; an empty/unset class %q-quotes to '' and is a harmless
+# no-op downstream (readPersonaContractBlock returns '' for an empty class).
+AGENT_CLASS_Q=$(printf '%q' "${MOSAIC_AGENT_CLASS:-}")
+
 if [ -n "$MOSAIC_RUNTIME_BIN_PREFIX" ]; then
-  PANE_SHELL_SNIPPET="export MOSAIC_AGENT_NAME=${AGENT_NAME_Q}; export PATH=\"${MOSAIC_RUNTIME_BIN_PREFIX}:\${PATH}\"; exec ${MOSAIC_AGENT_COMMAND}"
+  PANE_SHELL_SNIPPET="export MOSAIC_AGENT_NAME=${AGENT_NAME_Q}; export MOSAIC_AGENT_CLASS=${AGENT_CLASS_Q}; export PATH=\"${MOSAIC_RUNTIME_BIN_PREFIX}:\${PATH}\"; exec ${MOSAIC_AGENT_COMMAND}"
 else
-  PANE_SHELL_SNIPPET="export MOSAIC_AGENT_NAME=${AGENT_NAME_Q}; exec ${MOSAIC_AGENT_COMMAND}"
+  PANE_SHELL_SNIPPET="export MOSAIC_AGENT_NAME=${AGENT_NAME_Q}; export MOSAIC_AGENT_CLASS=${AGENT_CLASS_Q}; exec ${MOSAIC_AGENT_COMMAND}"
 fi
 
 mkdir -p "$MOSAIC_AGENT_WORKDIR"

packages/mosaic/framework/tools/fleet/test-start-agent-session.sh

@@ -104,6 +104,7 @@ PATH="$FAKE_BIN:$PATH" \
 MOSAIC_TMUX_SOCKET="$SOCKET3" \
 MOSAIC_AGENT_WORKDIR="$WORKDIR3" \
 MOSAIC_AGENT_RUNTIME="pi" \
+MOSAIC_AGENT_CLASS="code" \
 MOSAIC_RUNTIME_BIN="$FAKE_RUNTIME_BIN" \
 MOSAIC_AGENT_COMMAND="mosaic yolo pi --model openai-codex/gpt-5.5:high" \
 MOSAIC_HEARTBEAT_RUN_DIR="$HB_RUN_DIR3" \
@@ -127,6 +128,18 @@ echo "$all_args" | grep -qF "exec " || fail "pane command does not use exec"
 echo "$all_args" | grep -qF "mosaic yolo pi --model openai-codex/gpt-5.5:high" || \
   fail "pane command does not forward MOSAIC_AGENT_COMMAND with flags intact"
 
+# d) MOSAIC_AGENT_NAME and the per-agent MOSAIC_AGENT_CLASS must BOTH be exported
+#    INTO the pane. The pane inherits the tmux SERVER environment (not this
+#    script's env, nor the systemd unit's EnvironmentFile), so any per-agent var
+#    the launcher needs in-pane must be re-exported in the snippet. CLASS is
+#    load-bearing: the launcher composes the persona contract from
+#    process.env.MOSAIC_AGENT_CLASS, so a missing export silently drops the
+#    persona (regression guard for the A3a pane-propagation gap).
+echo "$all_args" | grep -qF "export MOSAIC_AGENT_NAME=" || \
+  fail "pane command does not export MOSAIC_AGENT_NAME into the pane"
+echo "$all_args" | grep -qF "export MOSAIC_AGENT_CLASS=code" || \
+  fail "pane command does not export MOSAIC_AGENT_CLASS into the pane (persona would silently drop)"
+
 # ── Test 4: when no extra runtime-bin dirs exist, exec still appears ───────────
 TMUX_ARGS_FILE2=$(mktemp)
 FAKE_BIN2=$(mktemp -d)