add bug-finding skill-writer helper

[sentry-junior]

we need a skill-writer helper tailored to warden's bug-finding skill workflow.

• use sentry's skill-writer as inspiration, but adapt it to warden's workflow rather than cloning it directly
• make source handling and traceability explicit so generated skills show where findings came from and how they were derived
• scope it around the concrete needs of building bug-finding skills, including the research and evidence flow those skills depend on

Action taken on behalf of David Cramer.