docs: improve cell barcode documentation

[nh13]

Summary

• Expanded the --cell-tag option description for both group and dedup commands to explain that reads at the same genomic coordinates are partitioned by cell barcode before UMI assignment/deduplication, preventing reads from different cells from being grouped together
• Added a "Cell Barcodes" section to the command-level documentation for both group and dedup explaining the partitioning behavior and noting that no correction is performed on cell barcodes

Test plan

• cargo check passes
• Verify fgumi group --help and fgumi dedup --help display the updated documentation

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 062a0356-d345-4deb-a557-d84730cafbee

📥 Commits

Reviewing files that changed from the base of the PR and between ec8c1ab and a7bb957.

📒 Files selected for processing (2)

• src/commands/dedup.rs
• src/commands/group.rs

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📝 Walkthrough

Walkthrough

Documentation-only updates to two CLI commands: MarkDuplicates and GroupReadsByUmi. Both expanded the --cell-tag help text to clarify that reads are partitioned by cell barcode before deduplication and grouping, preventing reads from different cells from being grouped together. New "Cell Barcodes" sections were added to both commands explaining this behavior and noting that no barcode correction or error-handling is performed. No functional code changes occurred.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'docs: improve cell barcode documentation' clearly and concisely describes the main change—expanding documentation for cell barcode handling across multiple commands.
Description check	✅ Passed	The description directly explains the changes made: expanded --cell-tag documentation and added Cell Barcodes sections to both group and dedup commands, matching the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch nh/cell-tag-docs

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.29%. Comparing base (ec8c1ab) to head (a7bb957).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #177   +/-   ##
=======================================
  Coverage   84.29%   84.29%           
=======================================
  Files         128      128           
  Lines       51978    51978           
=======================================
+ Hits        43814    43815    +1     
+ Misses       8164     8163    -1     

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[nh13]

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