chore: add Dependabot config for gradle, go, and github-actions

[AhmadAL-Quraan]

What

Adds .github/dependabot.yml, configuring automated dependency updates
for three ecosystems present in this repo:

• gradle (root Kotlin Multiplatform build)
• gomod (the separate tools/ Go module)
• github-actions (workflow action versions)

Why

Ran OpenSSF Scorecard against this repo and found Dependency-Update-Tool
at 0/10 — no update tool was configured. This adds one directly, and
should help two other low-scoring checks over time as Dependabot opens
update PRs: Pinned-Dependencies (0/31 GitHub Actions currently pinned
by SHA) and Vulnerabilities (9 known CVEs detected).

Weekly cadence, capped at 10 open PRs per ecosystem — matches OpenSSF's
recommended defaults.

Full Scorecard results (3.7/10 aggregate)

Score	Check	Summary
0/10	CII-Best-Practices	No OpenSSF best practices badge
0/10	Code-Review	Only 1/30 merged PRs went through review
0/10	Dependency-Update-Tool	No Dependabot/Renovate configured (this PR)
0/10	Fuzzing	No fuzzer integrations found
0/10	Maintained	Repo created within the last 90 days
0/10	Signed-Releases	No releases signed or with provenance
0/10	Token-Permissions	publish.yml/release-please.yml grant contents: write at workflow level
1/10	Pinned-Dependencies	0/31 GitHub-owned Actions pinned by SHA
1/10	Vulnerabilities	9 known CVEs in dependencies
4/10	Security-Policy	SECURITY.md exists, no linked disclosure content
6/10	Contributors	2 contributing orgs
9/10	Binary-Artifacts	One binary in source (gradle-wrapper.jar)
9/10	CI-Tests	26/27 merged PRs checked by CI
9/10	SAST	CodeQL detected, not run on 1/27 commits
10/10	Dangerous-Workflow	No dangerous patterns detected
10/10	License	MIT license file detected
10/10	Packaging	Releases packaged via GitHub Actions