docs: document historical Gemini session sync

[wey-gu]

Summary

• Document the new nmem t sync --from gemini-cli historical backfill flow.
• Clarify preview-first usage and project-scoped alternatives.

Verification

• Covered by parent/community validation for historical sync commands.
• Website docs build passed from the parent workspace.

Summary by CodeRabbit

• Documentation
  • Added guidance for performing historical backfill operations, including preview and apply steps.
  • Documented best practices for single-project synchronization.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a3aa81c5-388f-4f4e-baaa-a18400857e2e

📥 Commits

Reviewing files that changed from the base of the PR and between 4d00139 and 8e5e93e.

📒 Files selected for processing (1)

• README.md

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📝 Walkthrough

Walkthrough

Documentation added to README.md showing how to perform a historical backfill using nmem t sync from gemini-cli. The example covers previewing with --limit 20, applying with --apply, and using -p /path/to/project for targeted single-project sync.

Changes

Historical Backfill Sync Documentation

Layer / File(s)	Summary
Backfill sync workflow documentation README.md	New section documenting the two-step sync workflow: a preview step using --limit 20 to check results before applying with --apply, plus guidance on using -p /path/to/project for single-project sync.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 A guide for syncing deep in time,
With previews first—a cautious climb.
Limit twenty, then apply with care,
One project focused, smooth and fair.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'docs: document historical Gemini session sync' directly and clearly describes the main change: adding documentation for historical Gemini session synchronization, which matches the PR objectives and file changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch dev_0901

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