## 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`
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Co-authored-by: chatgpt-codex-connector[bot] <199175422+chatgpt-codex-connector[bot]@users.noreply.github.com>
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