## Why
Codex memory updates currently rely on instructions that tell agents to
create ad-hoc note files directly in the memory workspace. The memories
extension already has a `MemoriesBackend` abstraction for local storage
and future non-filesystem backends, so the ad-hoc note writer should
live behind that same interface instead of baking local filesystem
assumptions into the tool shape.
## What
- Adds a `memories/add_ad_hoc_note` tool to the existing memories tool
bundle.
- Extends `MemoriesBackend` with `add_ad_hoc_note` plus request/response
types so remote memory stores can implement the same operation later.
- Implements the local backend by creating append-only notes under
`extensions/ad_hoc/notes`.
- Validates the tool-provided filename contract
(`YYYY-MM-DDTHH-MM-SS-<slug>.md`), rejects path-like filenames, rejects
empty notes, and uses create-new semantics so existing notes are never
overwritten.
- Keeps memories tool contribution behind the existing commented-out
registration path; this defines the tool surface without newly exposing
it through app-server.
## Test Plan
- `just test -p codex-memories-extension`
## Why
The memories extension has several distinct responsibilities:
registering its prompt and tool contributors, enforcing local-memory
filesystem boundaries, implementing list/read/search behavior, and
wrapping that backend as extension tools. Those responsibilities were
concentrated in `lib.rs`, `local.rs`, and the tool modules, which made
follow-up work harder to review and risked growing files through
unrelated edits.
This PR reorganizes the crate so each responsibility has a narrower
owner while preserving the same extension entrypoint and memory tool
behavior.
## What Changed
- Moved extension lifecycle, prompt, and tool registration into
`src/extension.rs`, leaving `src/lib.rs` as the small crate entrypoint.
- Split `LocalMemoriesBackend` helpers into `local/list.rs`,
`local/path.rs`, `local/read.rs`, and `local/search.rs`.
- Centralized tool names and limits at the crate level, and kept the
backend and extension implementation crate-private.
- Made `memory_list`, `memory_read`, and `memory_search` tool executors
generic over `MemoriesBackend`, so tests can exercise the full executor
path without depending on tool internals.
- Consolidated and expanded memory extension tests in `src/tests.rs`,
including read/search tool output coverage, multi-query search, windowed
`all_within_lines`, and legacy `query` rejection.
## Testing
- Not run locally.
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