feat: Welcome first-time contributors with comment commands guide

[Ma77Ball]

What changes were proposed in this PR?

• Add .github/workflows/welcome-first-time-contributor.yml, which posts a single welcome comment listing the /take, /untake, /sub-issue, /unsub-issue, /parent-issue, /unparent-issue, /request-review, and /unrequest-review commands when a new contributor opens their first issue or PR.
• Detect first-time status with the GitHub search API (repo:<owner>/<repo> is:issue|pr author:<login>, treating total_count <= 1 as first time). This works uniformly for issues and PRs, whereas author_association reports NONE for someone opening their first issue, and would miss them.
• Trigger on pull_request_target (not pull_request) so PRs from forks still receive the welcome with a write-capable token; restrict permissions to issues: write and pull-requests: write.
• Make the workflow idempotent via a hidden HTML marker (<!-- texera:welcome-first-time-contributor -->) embedded in the welcome body and checked against existing comments before posting, so workflow re-runs and reopen races do not duplicate the welcome.

Any related issues, documentation, or discussions?

Closes: #5166

How was this PR tested?

• Validated the workflow YAML parses cleanly with python3 -c "import yaml; yaml.safe_load(open('.github/workflows/welcome-first-time-contributor.yml'))".
• Reviewed logic for the known edge cases: search indexing delay (the <= 1 check tolerates both 0, not yet indexed, and 1, only the new item), transient listComments failure (falls open rather than dropping a genuine first-timer's welcome), and Bot guard at the job-level if.
• Runtime behavior cannot be exercised end-to-end locally; will be observable on the next first-time issue or PR after merge.

Was this PR authored or co-authored using generative AI tooling?

Co-Authored with Claude Opus 4.7 in compliance with ASF Guidelines

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.55%. Comparing base (8a7366f) to head (ee7f848).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5222      +/-   ##
============================================
- Coverage     48.57%   48.55%   -0.02%     
+ Complexity     2379     2374       -5     
============================================
  Files          1042     1042              
  Lines         39971    39971              
  Branches       4252     4252              
============================================
- Hits          19415    19409       -6     
- Misses        19410    19412       +2     
- Partials       1146     1150       +4     

Flag	Coverage Δ
access-control-service	39.53% <ø> (ø)
agent-service	33.76% <ø> (ø)
amber	51.53% <ø> (-0.05%)	⬇️
computing-unit-managing-service	0.00% <ø> (ø)
config-service	0.00% <ø> (ø)
file-service	37.99% <ø> (ø)
frontend	40.01% <ø> (-0.01%)	⬇️
python	90.51% <ø> (+0.04%)	⬆️
workflow-compiling-service	56.81% <ø> (ø)

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