feat(hooks): opt-in agent-type scoping for SubagentStart injection (#… (#522)

* feat(hooks): opt-in agent-type scoping for SubagentStart injection (#506)

The SubagentStart hook injected the ~1,300-token persona into every
subagent, including read-only search agents where it is dead weight.
Consumers could not narrow a plugin-provided hook from their own
settings, so the scoping has to live here.

Add an opt-in allowlist: PONYTAIL_SUBAGENT_AGENTS (comma-separated env
var) or a subagentAgents array in config.json, matching getDefaultMode
precedence. The hook now reads agent_type from stdin and skips
injection only on a positive mismatch; unconfigured or missing
agent_type keeps the inject-everywhere behavior from issue #252. Stdin
reading follows the never-hang contract from issue #443.

Closes #506

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Address review: regex matcher, stdin-free default path, exit on skip

Rework the SubagentStart scoping per review on #522:

- Replace the comma-separated PONYTAIL_SUBAGENT_AGENTS allowlist with a
  regex PONYTAIL_SUBAGENT_MATCHER, matching the reviewed API on #507 and
  the PONYTAIL_DEFAULT_MODE naming convention.
- Default path (no matcher) injects synchronously and never reads stdin,
  fixing the #443-class Windows regression where the persona was delayed
  until a 1s timeout.
- On a definite agent_type mismatch the hook now process.exit(0)s
  immediately instead of lingering until the fallback timer.
- Drop the config-file (subagentAgents) source, so no config read on the
  subagent hot path; env-only keeps it lazy (YAGNI).
- Revert the unrelated tests/hooks-windows.test.js change; the scoped
  path mirrors the mode-tracker never-hang contract already tested there.

Fail open on unparseable input, missing agent_type, invalid regex, or
timeout so scoping never silently drops the persona (#252).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Emeriko <dietrich.gebert@gmail.com>