Feature Request: --message-file flag for openclaw message send

Problem

When AI agents generate completion reports containing code blocks, JSON, multiline content, $ references, backticks, or quotes, passing them via shell expansion to --message causes:

• Bad substitution errors (line 1: bad substitution)
• Mangled or truncated messages
• Silent send failures

Current workaround patterns are fragile and error-prone:

# Dangerous — shell interpolates $, backticks, quotes
openclaw message send --message "$(cat report.txt)"

# Also fragile — heredoc still subject to expansion
openclaw message send --message "$(cat << 'EOF'
$(cat report.txt)
EOF
)"

This means agent completion notifications — the primary way background tasks report back to users — are unreliable whenever output contains code.

Proposed Solution

Add a --message-file <path> flag to openclaw message send:

openclaw message send --channel telegram --target 8611381068 --message-file /tmp/report.txt

Acceptance Criteria

• Reads file as UTF-8
• Preserves multiline content exactly — no shell interpolation
• Supports all existing routing flags (--channel, --target, --account, --reply-to, --thread-id)
• Mutually exclusive with --message (error if both provided)
• Handles max length: if file exceeds platform message limit, truncate with a trailing note ([truncated]) rather than silently dropping or failing
• Returns nonzero exit code on send failure
• Prints a clear error if the file does not exist or is not readable

Use Case

Agent completion notification pattern:

# Agent writes report to temp file
cat > /tmp/agent-report.txt << 'REPORT'
Done: Fixed 4 critical defects in LiveShiftForegroundService.kt
- Timer replaced with Handler (tick loop survives exceptions)
- foregroundServiceType changed from dataSync to health (6h limit removed)
- PARTIAL_WAKE_LOCK acquired on shift start (Doze fix)
- startForeground() moved to onCreate() (ANR window closed)
REPORT

# Safe send — no shell expansion risk
openclaw message send --channel telegram --target 8611381068 --message-file /tmp/agent-report.txt

Context

Discovered during Log-it Android service fix where two background Claude Code agents completed their work but failed to send Telegram completion notifications due to shell expansion issues in the completion prompt.