Add OpenAPI path conflict detection to CI (#717)

Introduces a Go script that analyzes HTTP paths in the OpenAPI spec and
fails if any literal path segments conflict with parameterized segments
at the same position (e.g. /items/pause vs /items/{id}).

**NOTE:** Also removed the `execute-multi-operation` annotation which
was broken due to a conflict with the start workflow path. Removed the
experimental annotation from that API.

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Co-authored-by: Claude Opus 4.6 <[email protected]>

Author: bergundy