fix(plugin-dev): prevent validate-agent.sh exiting on first warning

[erichanwang]

Summary

Follow-up to #76985.

Prevent validate-agent.sh from terminating after the first warning or error when running under set -e.

Problem

The validator increments counters using:

((error_count++))
((warning_count++))

In Bash these expressions return a non-zero exit status when the previous value is zero, which can trigger errexit and stop validation before the remaining checks and summary are reached.

PR #76985 intentionally left this issue out of scope.

Changes

Replace postfix increments with:

((error_count += 1))
((warning_count += 1))

This preserves the counter values while allowing validation to continue through all validation checks.

Verification

• Reproduced the premature exit before the fix.
• Verified the validator now reaches the final summary.
• bash -n plugins/plugin-dev/skills/agent-development/scripts/validate-agent.sh
• git diff --check

Scope

This PR only changes counter increments to avoid unintended errexit behavior. No validation rules or output were modified.

🤖 Generated with Claude Code

https://claude.ai/code/session_016A6KkbNjaUq7QKa99jAcfW