Summary

Closes wizard-leftovers follow-up: CLI mm config unset/reset.

PR #258 closed the fragment/env/factory drag-in for new saves, but
left a known limitation — historical config.json entries that don't
match the local comparand (e.g., machine-A paths in memory_dirs
carried to machine-B, or mmr.enabled=false shadowing a config.d
fragment that sets it true) stay pinned until actively reset. The
existing escape hatches were hand-edit or mm init --fresh, both too
heavy for a single stale key.

mm config unset <key> is the targeted affordance. Distinct from
mm init --fresh (single-key vs bulk; no backup vs backup;
idempotent scripting vs wizard re-run). Both commands stay.

Behaviour (6 locked decisions)

1. Ambient cleanup: no. Unset touches exactly the named keys.
   Default-equal leftovers in other fields get pruned on the next
   normal save via fix(config): delta-only save against comparand to close fragment/env drag-in #258's delta-only write.
2. Valid-key set = MUTABLE_FIELDS ∪ _EXTRA_MUTATION_FIELDS.
   indexing.memory_dirs is accepted (removal isn't a mutation, and
   the migration case is exactly what motivates this command) even
   though mm config set memory_dirs … stays rejected.
3. Already-unset canonical key → exit 0 + (already at default).
   Idempotent re-runs are safe.
4. Unknown key → exit 1 with a difflib suggestion (cutoff 0.7)
   when a canonical key is nearby, otherwise just Skipped.
5. Malformed config.json → exit 1, no auto-recovery, message
   points at mm init --fresh. Unset is raw-edit semantic; hiding
   malformed state behind a backup would confuse.
6. Empty config.json after removal is deleted with a note line
   (load_config_overrides short-circuits on missing file).

_EXTRA_MUTATION_FIELDS (currently just indexing.memory_dirs)
triggers a domain warning pointing at dedicated endpoints
(mm memory-dirs list / mm index) after successful removal.

Output matrix

Multi-key input is best-effort: iterate, collect Removed /
Already-unset / Skipped buckets, emit per-key lines. Exit 0 if no
skips, 1 otherwise.

_atomic_write_json

Introduced in config.py next to _json_default — tempfile in
path.parent + os.replace, with tmp cleanup on failure. Used by
unset as initial consumer. save_config_overrides and
cli/init_cmd.py's --fresh write path still call write_text
directly; migrating them is the remaining wizard-leftovers follow-up
on "--fresh write-after-backup atomicity" and reduces to swapping
those two call sites once this PR lands.

Tests (13 in TestConfigUnset)

Full output matrix, empty-file deletion, _EXTRA_MUTATION_FIELDS
allowance + domain warning, malformed JSON handling, multi-key
best-effort, atomic-write-preserves-original-on-os.replace-failure

• cleans-up-tmp-on-success, plus a fragment-reappearance end-to-end
  regression guard (fragment sets mmr.enabled=true, config.json
  pins false, unset → reload → fragment layer wins).

Docs

• docs/guides/configuration.md: new "Removing individual
  overrides" subsection; the existing machine-migration section
  now lists mm config unset indexing.memory_dirs as Option 1.
• docs/guides/getting-started.md + docs/guides/user-guide.md:
  cheatsheets updated.

Test plan

• uv run pytest -m "not ollama" -q — 1712 passed
• uv run ruff check packages/memtomem/src packages/memtomem/tests
• uv run ruff format --check packages/memtomem/src packages/memtomem/tests
• uv run mypy packages/memtomem/src — success, no issues
• 13 new TestConfigUnset tests all green