feat(mcp,session): provide session registry with mcp resource endpoint

[radimsem]

Summary

Closes #112

Self-review

Touched

Pipeline

• pkg/parser/ — fuzz target covers any new shape
• pkg/transformer/ or pkg/diff/ — ID stability / hash inputs unchanged
• pkg/emitter/ — exactly one snapshot per write
• pkg/store/ + migrations/ — FTS5 triggers in sync; add-store-query followed
• pkg/query/ or pkg/compiler/ — token budget honored

MCP surface

• pkg/mcp/ — skills/remind (read) or skills/memoize (write) updated
• pkg/temperature/ — both public skills reflect new values

Edges

• cmd/remindb/ — README CLI section updated
• internal/ignore/, internal/tempfile/, internal/bench/ — relevant fuzz / fixture / scenario added
• plugins/<agent>/ — manifest version bumped if shipping

Process

• Tested manually via CLI or local MCP plugin install
• make test-all + make fuzz green