feat: adding support for Node 24 and Node 26

[erunion]

What

Adds Node 24 (Active LTS) and Node 26 (current) support to @readme/nodegit: tests run on and prebuilds ship for Node 22, 24, and 26.

Changes

• Swap @axosoft/nan for upstream nan@^2.28.0 via npm alias. The fork is stale at 2.22.0-gk.1; upstream nan ≥2.27.0 is required for Node 26 / V8 14.x. The alias keeps the require('@axosoft/nan') include-path lookup in the binding.gyp template working unchanged, minimizing the diff vs upstream nodegit for future merges. The fork's -gk patches are Electron/cppgc-specific and unused by this repo.
• Fix V8 14.x removal: v8::Context::GetIsolate() no longer exists on Node 26. Replaced the Nan::GetCurrentContext()->GetIsolate() pattern with v8::Isolate::GetCurrent() at four JS-thread call sites in generate/templates/. This was the only compile fix Node 26 needed.
• Widen engines to ^22 || ^24 || >=26 (even/LTS lines only).
• CI matrices: tests.yml and publish.yml now run Node 22/24/26. Removed hardcoded CXXFLAGS: -std=c++17 env overrides — binding.gyp already emits the correct -std=c++<ver> per running Node (C++20 on 24/26), and the override would fight it.
• Publish artifact merging generalized from hardcoded -node22 artifact names to a glob over all prebuild-* artifacts. ABI-tagged prebuild filenames don't collide, so all versions merge into the same platform dirs.
• Alpine Dockerfile: node:${NODE_VERSION}-alpine3.19 has no 24/26 tags; moved to the floating -alpine tag, bumped the default from 20 to 22, and dropped the -std=c++17 portion of CXXFLAGS so the gyp-derived standard applies.
• Docs (README, CLAUDE.md) updated to 22/24/26.

Verification (local, macOS arm64)

Clean build (rm -rf build node_modules && npm install) + full mocha suite on each version:

Node	Build	Tests
22.21.0 (regression)	✅	420 passing, 12 pending, 7 failing; lint ✅
24.11.0	✅	420 passing, 12 pending, 7 failing
26.5.0	✅	420 passing, 12 pending, 7 failing

The 7 failures are byte-identical across all three versions and are pre-existing local-environment issues, not regressions: three require SSH test credentials (test/id_rsa is only checked in encrypted; CI decodes it) and four fail on local git default-branch config (no reference found for shorthand 'master'). CI has the SSH/git setup these need.

Before release

• Full CI matrix green on this PR
• Optional musl smoke test: docker build --build-arg NODE_VERSION=26 -f scripts/Dockerfile.alpine .
• After merge: workflow_dispatch run of publish.yml and confirm find ./prebuilds shows 18 .node files (3 versions × 6 platform/libc targets) before tagging v3.3.0

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