Summary

Ad-hoc memory notes are written under memories/extensions/ad_hoc/, but the consolidation agent only knows how to interpret an extension when the extension folder has an instructions.md. Seed those instructions from the memories write pipeline so an enabled memories startup creates the expected ad-hoc extension layout automatically.

This also moves extension-specific write behavior behind a dedicated memories/write/src/extensions/ module. ad_hoc owns the seeded instructions template, while the existing resource-retention cleanup lives in its own prune module so future memory extensions can add their own write-side setup without growing a flat helper file.

Changes

• Seed memories/extensions/ad_hoc/instructions.md during eligible memory startup without overwriting an existing file.
• Store the ad-hoc instructions template under memories/write/templates/extensions/ad_hoc/, keeping ownership in codex-memories-write.
• Split memory extension support into extensions::ad_hoc and extensions::prune.
• Keep the existing old-resource pruning behavior unchanged.

Verification

• cargo test -p codex-memories-write
• bazel build //codex-rs/memories/write:write