feat(secret-serialization): Add skill for secrets leaked via serialization

[geoffg-sentry]

Adds secret-serialization, a review skill for credentials that leak through generated serialization: dataclass repr and asdict, attrs, pydantic model_dump, JSON.stringify, util.inspect, structured logs, and tracing spans.

These leaks need two changes that each look safe alone. One stores a credential on a type that serializes itself, for example a dataclass field without field(repr=False). The other serializes whole objects or every kwarg into telemetry, for example str(value) over tool-call kwargs written to a span attribute. They are usually written months apart, so diff-only review passes both. The skill reports either side on its own and searches the whole repository for the other side before choosing severity. It does not downgrade a finding because one half predates the diff.

Direct logging of a secret stays with security-review. This skill is opt-in, and repos add it through Warden with remote = "getsentry/skills".

Evaluation

Run through Warden (pi runtime, openrouter/x-ai/grok-4.5, effort high) against the internal repository where the original leak happened:

• Incident diff (credential fields added to dataclass RPC clients; the str(value) span sink was already on the default branch): high findings on both fields in two runs. Each finding traced tool kwarg, then tracing decorator, then str(value), then span attribute.
• Same diff with field(repr=False): no findings.
• Unrelated diff adding seven *_tokens usage-count fields: no findings.

Details and open gaps are in SOURCES.md. Known noise: call-site hunks that pass the credential into a constructor produce duplicate findings for the same field.

allowed-tools must be space-delimited for Warden. With Read, Grep, Glob, Warden drops Read, and Grep,, so the agent could not read files or search code, and the incident only produced medium findings. Other skills in this repo that use the comma form, such as gha-security-review, likely have the same problem under Warden.

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