* feat: emit structured runtime metadata
* fix: avoid subagent import cycle in replay gateway
* fix: preserve legacy subtask result parsing
* refactor: tighten runtime metadata contracts
* fix(middleware): keep recovery hint on task exception wrapper content
The structured-metadata stamp overwrote the wrapper text with the bare
task-failure message, dropping the model-facing 'Continue with available
context, or choose an alternative tool.' guidance that every other tool
exception keeps. Append the shared hint after the formatted message.
* fix(subagents): require lowercase hex for result_sha256 reader
Length-only validation accepted any 64-char string; a faulty serializer
or relaying wrapper could store a non-digest value in the delegation
ledger. Enforce the producer's hexdigest shape with a fullmatch.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
