Dependency + CI maintenance: SciMLBase 3 support, CompatHelper fix, drop duplicate Dependabot

[mmogib]

Summary

Maintenance PR consolidating today's dependency/CI fixes (no source-behavior changes):

1. Support SciMLBase 3.x (3728d84) — widen compat "2.53" → "2.53, 3". CI-verified: the full 262-test suite + docs build pass against SciMLBase 3.x across Julia 1.10/1.12/pre on x64+x86 (originally surfaced as CompatHelper CompatHelper: bump compat for SciMLBase to 3, (keep existing compat) #21). The package's SciMLBase surface (NonlinearProblem, isinplace, ReturnCode, NLStats, build_solution, __solve) is unaffected by the 2.x→3.x major bump.

2. Fix recurring CompatHelper 403 (a483f6c) — the repo policy Allow GitHub Actions to create and approve pull requests was disabled, blocking PR creation (the SSH-key push half always worked). Enabled it at the repo level; this commit additionally scopes the workflow's GITHUB_TOKEN to least privilege (contents + pull-requests write only).

3. Drop duplicate Dependabot julia ecosystem (f43da65) — CompatHelper and Dependabot were both managing Julia deps (duplicate SciMLBase PRs build(deps): Update SciMLBase requirement from 2.53 to 2.53, 3.16 #16 ≡ CompatHelper: bump compat for SciMLBase to 3, (keep existing compat) #21). Standardize on CompatHelper (registry-aware, the ecosystem norm); keep the github-actions ecosystem.

Supersedes and closes bot PRs #16–#21.

Note: this updates main's source only — the registry's published v0.3.2 keeps "2.53". SciMLBase 3 support reaches users at the next release.

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.81%. Comparing base (114f7ef) to head (f43da65).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   90.81%   90.81%           
=======================================
  Files          12       12           
  Lines         675      675           
=======================================
  Hits          613      613           
  Misses         62       62           

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[mmogib]

Closing — DFMethods releases through release.sh, whose PR step reuses any open feature-dev → main PR. Leaving this open would make the eventual release PR inherit this maintenance title instead of Release vX.Y.Z. The three commits stay on feature-dev as the [Unreleased] entry-in-waiting and will ship to main via the next release. Not deleting the branch (it IS feature-dev).