feat: overhaul contributor tracking (workflow, list refresh, six badge types)

[edkerk]

Main improvements in this PR:

This overhauls how contributors are tracked (addresses #414) and refreshes the list, which had not been updated since November 2023.

Adding contributors (#414). The all-contributors App can only open pull requests against the default branch main (app#243, open since 2019); credits added there are lost when main is rebuilt from develop at release.

• Add .github/workflows/add-contributor.yml. A maintainer comments @all-contributors please add @user for bug, code on any issue or pull request; the workflow runs the all-contributors CLI and opens the pull request against develop. Runs on the default GITHUB_TOKEN, write-access only.
• Document the flow in .github/CONTRIBUTING.md and add a reminder to the pull request template.

Refreshed contributor list.

• Add 12 contributors active since the list was last updated, and restore Jiahao Luo, who was shown in the README but missing from .all-contributorsrc (a main/develop divergence artifact). Credit recent work by edkerk, Devlin-Moyer and johan-gson. Total 31 → 43.
• Consolidate the 16 contribution types in use to six (bug, research, code, review, doc, ideas), combining overlapping credits so the grid is readable.
• Show 10 contributors per row at a smaller image size (80 → 64 px). Regenerating the grid also repairs 105 corrupted emoji (stored as byte 0x1A) and drops a duplicate Christoff1993 cell.

Housekeeping. Remove the retired Gitter references (README badge, CONTRIBUTING, .standard-GEM.md).

Two one-time repository settings (admin) are needed for the workflow to take over from the App: uninstall the "All Contributors" App, and enable Settings → Actions → General → "Allow GitHub Actions to create and approve pull requests".

I hereby confirm that I have:

• Tested my code on my own computer for running the model
• Selected develop as a target branch
• Any removed reactions and metabolites have been moved to the corresponding deprecated identifier lists

[github-actions]

This PR has been automatically tested with GH Actions. Here are the results of the quality-control tests:

MACAW: dead-end and duplicate tests

Starting dead-end test...
 - Found 1384 dead-end metabolites.
 - Found 1141 reactions incapable of sustaining steady-state fluxes in either direction due to these dead-ends.
 - Found 1369 reversible reactions that can only carry steady-state fluxes in a single direction due to dead-ends.
Starting duplicate test...
 - Skipping redox duplicates because no redox_pairs and/or proton_ids were provided.
 - Found 377 reactions that were some type of duplicate:
   - 0 were completely identical to at least one other reaction.
   - 13 involve the same metabolites but go in the opposite direction or have the opposite reversibility as at least one other reaction.
   - 377 involve the same metabolites but with different coefficients as at least one other reaction.

Mass and charge balance

Unbalanced reactions (excluding boundary and biomass): 277 (87 mass, 240 charge)

More detailed output is committed to data/testResults/ (macaw_results.csv and balance_results.csv).

Note: In the case of multiple test runs, this post will be edited.

[mihai-sysbio]

Well done @edkerk 🙌🏻