fix: guardrail hooks fail-open instead of fail-closed

[kwacky1]

Problem

The guardrail hooks in copilot-plugin/hooks/_common.sh use set -e, which causes the script to exit with a non-zero code if anything unexpected happens before reaching the toolName != bash → exit 0 check.

When the Copilot CLI invokes these hooks for non-bash tools (e.g. Zendesk MCP, GitHub MCP, or any other MCP tool call), the hook errors out rather than returning a clean exit. The CLI interprets a hook error as a denial, which blocks all tool calls — including reads, shell commands, and MCP calls.

Fix

1. Remove set -e
2. Add trap 'exit 0' ERR — fail-open on unexpected errors
3. Add early validation: if input is empty or not valid JSON, exit 0 immediately

These hooks only guard against destructive bash operations — they should never interfere with non-bash tools.

Testing

# Non-bash tool → allowed
echo '{"toolName":"get_ticket_comments","toolArgs":{"ticket_id":"123"}}' | bash hooks/guard-destructive-ops.sh
# exit 0 ✅

# Empty input → allowed
echo '' | bash hooks/guard-destructive-ops.sh
# exit 0 ✅

# Safe bash → allowed
echo '{"toolName":"bash","toolArgs":{"command":"echo hello"},"cwd":"/tmp"}' | bash hooks/guard-destructive-ops.sh
# exit 0 ✅

# Destructive bash → denied (correctly)
echo '{"toolName":"bash","toolArgs":{"command":"./delete-a-repo.sh"},"cwd":"/tmp"}' | bash hooks/guard-destructive-ops.sh
# outputs deny JSON ✅

Fixes #579

[gm3dmo]

Looks good to me.