feat: use git-backed workspace diffs for memory consolidation (#18982)

## Why

This PR make the `morpheus` agent (memory phase 2) use a git diff to
start it's consolidation. The workflow is the following:
1. The agent acquire a lock
2. If `.codex/memories` does not exist or is not a git root, initialize
everything (and make a first empty commit)
3. Update `raw_memories.md` and `rollout_summaries/` as before.
Basically we select max N phase 1 memories based on a given policy
4. We use git (`gix`) to get a diff between the current state of
`.codex/memories` and the last commit.
5. Dump the diff in `phase2_workspace_diff.md`
6. Spawn `morpheus` and point it to `phase2_workspace_diff.md`
7. Wait for `morpheus` to be done
8. Re-create a new `.git` and make one single commit on it. We do this
because we don't want to preserve history through `.git` and this is
cheap anyway
9. We release the lock
On top of this, we keep the retry policies etc etc

The goals of this new workflow are:
* Better support of any memory extensions such as `chronicle`
* Allow the user to manually edit memories and this will be considered
by the phase 2 agent
 
As a follow-up we will need to add support for user's edition while
`morpheus` is running

## What Changed

- Added memory workspace helpers that prepare the git baseline, compute
the diff, write `phase2_workspace_diff.md`, and reset the baseline after
successful consolidation.
- Updated Phase 2 to sync current inputs into `raw_memories.md` and
`rollout_summaries/`, prune old extension resources, skip clean
workspaces, and run the consolidation subagent only when the workspace
has changes.
- Tightened Phase 2 job ownership around long-running consolidation with
heartbeats and an ownership check before resetting the baseline.
- Simplified the prompt and state APIs so DB watermarks are bookkeeping,
while workspace dirtiness decides whether consolidation work exists.
- Updated the memory pipeline README and tests for workspace diffs,
extension-resource cleanup, pollution-driven forgetting, selection
ranking, and baseline persistence.

## Verification

- Added/updated coverage in `core/src/memories/tests.rs`,
`core/src/memories/workspace_tests.rs`, `state/src/runtime/memories.rs`,
and `core/tests/suite/memories.rs`.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/git-utils/README.md

@@ -3,9 +3,11 @@
 Helpers for interacting with git, including patch application and worktree
 snapshot utilities. The crate also exposes a lightweight baseline API for
 internal directories that use git only as a resettable diff mechanism:
-`reset_git_repository` replaces `root/.git` with a fresh one-commit baseline,
-and `diff_since_latest_init` returns structured file changes plus a unified
-diff from that baseline to the current directory contents.
+`ensure_git_baseline_repository` preserves a usable `root/.git` baseline or
+creates one when it is missing or unusable, `reset_git_repository` replaces
+`root/.git` with a fresh one-commit baseline, and `diff_since_latest_init`
+returns structured file changes plus a unified diff from that baseline to the
+current directory contents.
 
 ```rust,no_run
 use std::path::Path;