chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/alecthomas/chroma/v2/AGENTS.md

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+Chroma is a syntax highlighting library, tool and web playground for Go. It is based on Pygments and includes importers for it, so most of the same concepts from Pygments apply to Chroma.
+
+This project is written in Go, uses Hermit to manage tooling, and Just for helper commands. Helper scripts are in ./scripts.
+
+Language definitions are XML files defined in ./lexers/embedded/*.xml.
+
+Styles/themes are defined in ./styles/*.xml.
+
+The CLI can be run with `chroma`.
+
+The web playground can be run with `chromad --csrf-key=moo`. It blocks, so should generally be run in the background. It also does not hot reload, so has to be manually restarted. The playground has two modes - for local development it uses the server itself to render, while for production running `just chromad` will compile ./cmd/libchromawasm into a WASM module that is bundled into `chromad`.