feat(ponytail): SubagentStart agent_type regex matcher

[getappz]

• Add subagent_should_inject() — reads PONYTAIL_SUBAGENT_MATCHER env var
• Case-insensitive regex match against agent_type from SubagentStart stdin
• Fail-open: invalid regex, missing agent_type, stdin timeout → inject anyway
• Default path (no matcher): inject everywhere, no stdin read
• 1s stdin timeout, never blocks
• Matches upstream ponytail#507 design
• Closes ponytail PR audit ticket [ponytail#507] SubagentStart hook: agent_type regex matcher via PONYTAIL_SUBAGENT_MATCHER #62

Summary by CodeRabbit

• New Features

  • Added configurable matching for when startup hooks are inserted, based on runtime input and an environment setting.
  • Hook handling now supports case-insensitive matching and safer fallback behavior when input or configuration is unavailable.

• Bug Fixes

  • Improved reliability by avoiding missed hook injection when input can’t be read, JSON is invalid, or the match pattern is unusable.

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• src/cli/ponytail.rs

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