chore: release v0.2.87

[AntoineToussaint]

Publishes the SourceDir-scoped immutable revision manifest fix from #214 for Codefly language agents and Mind production-session consumption.

Validation:

• fix(code): scope revision manifests to source root #214 focused go test ./code passed;
• full Core suite passed except a host Git signing-agent configuration error; the exact affected Python runner test passed with commit/tag signing disabled;
• nested-root regression proves worktree/revision path parity and exact content identities;
• go test ./version -count=1.