datasets: seed the first two agent-trace entries

[gautamgb]

Summary

The follow-up promised in #62: the two seed entries and index.csv, now that the schema and validator have merged.

• DSGAI-TRACE-mcp-declared-effect-flip-stale-autoapproval - a tool re-declares destructiveHint: true after an approval policy has already cached it into an auto-approve class, and the next call runs destructively with no human asked.
• DSGAI-TRACE-mcp-output-schema-expansion-context-overshare - a tool's output schema widens under a stable server identity, and a schema-agnostic context append carries three unentitled fields into the model context, the user-facing answer, and the observability sink.

Both are derived_from_public_documentation: fully synthetic sessions built around a precondition the MCP specification itself establishes (annotations untrusted from an untrusted server, no version field on a tool definition, tools/list_changed as a SHOULD, outputSchema obligations attaching to whatever schema is current), plus CVE-2025-54136 for the first entry, with the entry stating plainly what the CVE does and does not establish about the non-adversarial mechanism it illustrates.

index.csv is rebuilt against main as it stands now, so it includes #67's entry alongside these two - three rows total.

Verification

$ python validate.py
OK: 3 entries + example.json validated against schema.

$ python build_index.py
Wrote index.csv with 3 entries.

Rebased on the current main tip in a clean worktree (not cherry-picked over a stale base), so there's nothing here that predates #67's entry or the merged schema.