Consolidate six GitHub Actions workflows into a matrix workflow

[kilodesodiq-arch]

Problem Statement. .github/workflows/ has six jobs covering backend, AI service,
contract, ocr-regression, testnet-smoke, ci-python-tests. They share installs, caches,
and patterns.

Technical Context.
.github/workflows/*.

Expected Outcome.

• ci.yml matrix job covers backend (npm), ai-service (pip), contract (cargo), frontend (pnpm),
  mobile (pnpm).
• Specialty jobs (ocr-regression, testnet-smoke) remain separate due to runtime
  needs.

Acceptance Criteria.

• Single pnpm install --frozen-lockfile runs once.

Files or modules likely to be affected.
.github/workflows/ci.yml (new),
retirement of the six existing workflows.

Difficulty. Medium
Estimated effort. L

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Backlog item #97 from `docs/maintainer-issue-backlog.md.

[Chigybillionz]

Hi, I’ve reviewed this issue and I’m confident I can fix it. I’d like to take it on.

[Heazzy500]

Hi, I’d appreciate the opportunity to contribute to this issue. I’m detail-oriented and committed to delivering quality work.

[Ayinkx1]

Hi i would like to work on this issue please assign to me
I can do it perfectly

[emirykl]

Hi I want to take this issue