Remove exec agent — replaced by OpenClaw skill + vault API (#722)

## What

Removes the exec agent (PR #697). Its functionality is replaced by:

1. **OpenClaw skill** — teaches any OpenClaw instance to be the factory's face
2. **Vault API** — structured interface for proposals, approvals, rejections

The exec agent was rebuilding OpenClaw in bash. Every piece has a native OpenClaw equivalent:
- CHARACTER.md → SOUL.md
- exec/MEMORY.md → MEMORY.md
- exec-session.sh → session management
- exec-briefing.sh → heartbeats/cron
- Matrix dispatch → channel plugins

## Why

Prudence isn't a separate agent. She's what OpenClaw becomes when it has the disinto skill. One LLM, one vault API, no LLM-to-LLM.

## Related

- #721 — remove escalation, route through vault
- #709 — skill registry research
- #466 — example project (vault should have handled this, not escalation)

Co-authored-by: openhands <openhands@all-hands.dev>
Reviewed-on: https://codeberg.org/johba/disinto/pulls/722

lib/matrix_listener.sh

@@ -341,23 +341,6 @@ Interpret this response and decide how to proceed."
           fi
         fi
         ;;
-      exec)
-        # Route message to exec session — spawn on demand if needed
-        EXEC_PROJECT=$(awk -F'\t' -v id="$THREAD_ROOT" '$1 == id {print $5}' "$THREAD_MAP" 2>/dev/null || true)
-        EXEC_PROJECT="${EXEC_PROJECT:-${PROJECT_NAME:-disinto}}"
-        EXEC_TOML="${FACTORY_ROOT}/projects/${EXEC_PROJECT}.toml"
-        [ -f "$EXEC_TOML" ] || EXEC_TOML=""
-
-        # Delegate to exec-inject.sh (handles spawn + inject + capture + Matrix post)
-        nohup bash "${FACTORY_ROOT}/exec/exec-inject.sh" "$SENDER" "$BODY" "$THREAD_ROOT" \
-          "$EXEC_TOML" >> "$LOGFILE" 2>&1 &
-        log "exec message from ${SENDER} dispatched to exec-inject.sh"
-
-        if ! grep -qF "$THREAD_ROOT" "$ACKED_FILE" 2>/dev/null; then
-          matrix_send "exec" "✓ Message forwarded to executive assistant" "$THREAD_ROOT" >/dev/null 2>&1 || true
-          printf '%s\n' "$THREAD_ROOT" >> "$ACKED_FILE"
-        fi
-        ;;
       *)
         log "no handler for agent '${AGENT}'"
         ;;