Fix #394: require evidence fields in decompose_feature bug guidance

[yxlyx]

Summary

• update decompose_feature guidance so agent-discovered bugs/regressions explicitly require Exact repro, Observed result, Expected result, Nearby passing checks, Acceptance criteria, and Non-goals
• keep manual create_issue / create_issues_batch filing distinct from the stricter agent-discovered bug guidance
• add a src/tools.zig regression test that asserts the required evidence fields remain in the emitted instructions

Testing

• devswarm_run_explorer confirmed the updated guidance and regression test are present in src/tools.zig
• zig test src/tools.zig -fno-emit-bin reached the file but could not run standalone because the ad-hoc invocation lacks the repo's mcp module wiring
• full zig build test is still blocked on this machine by the existing macOS Zig linker environment

Closes #394