[PUP-528] feat: add configurable global sub-agent recursion limit

[thomwebb]

Jira

https://jira.walmart.com/browse/PUP-528

Summary

• add a global subagent_recursion_limit setting with a default of 4
• enforce the limit at the shared invocation seam for both sub-agent tools
• preserve the stricter existing GPT-5.6 single-level recursion policy
• expose the setting through /set; 0 disables sub-agent invocation
• localize user-facing recursion errors and add focused boundary/config coverage

Closes #660

Semantics

• main agent: depth 0
• directly invoked sub-agent: depth 1
• default limit 4: depths 1-4 are allowed; creating depth 5 is blocked
• invalid or negative values fall back to 4

Validation

• ruff check --fix .
• ruff format .
• full clean-environment suite: 12,436 passed, 81 skipped, 1 xpassed
• adversarial review: LGTM
• final post-rebase review: LGTM

GitHub issue

#660