* fix(subagents): reject Windows reserved device names in managed subagent creation
* Reject Windows reserved device names (CON, PRN, AUX, NUL, COM1-9, LPT1-9)
in normalize_managed_subagent_name when for_creation=True via
windows_incompatible_segment.
* Validate subagent names on creation in FileManagedSubagentStore.create
and /api/subagents POST gateway router before attempting disk writes.
* Keep read, update, and delete paths permissive for backward compatibility.
* Add unit tests for store validation and router endpoints.
* Update CHANGELOG.md under Fixed.
* fix(subagents): reject reserved names in SQL managed-subagent create
Apply the same for_creation check used by the file store so both
backends refuse Windows device names. Pin mixed-case Nul in tests and
import the shared normalizer at module level.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
