Part of #489. Depends on #490, #491 shipping in v0.8.9.

Summary

Support @path syntax inside memory files (relative, ~, absolute) so users can break long files apart and share common rules.

Reference

reference-cc/src/utils/claudemd.ts:448-501.

Behavior

• @./shared/style.md, @~/dotfiles/agents.md, @/abs/path/file.md all resolved.
• HashSet<PathBuf> tracks already-included files to break cycles.
• Inline at the import site, with a <imported from=...> wrapper.
• Surface error in transcript if file missing or unreadable (don't silently swallow).

Acceptance

• Two-level imports work (A → B → C).
• Cycle (A → B → A) is broken and surfaces a friendly warning.
• File-size cap applied per file, not just per top-level.

Effort: M (~250 LoC). Hardest piece in the EPIC.