Migrate CI from hand-crafted .taskcluster.yml to taskgraph

[JohanLorenzo]

Summary

• Replaces the hand-crafted v1 `.taskcluster.yml` with a taskgraph-based setup
• Adds in-tree Docker images (`python`, `taskboot`, `node`) for all CI tasks
• Custom parameters module sets `channel` and `backend_url` from branch/tag

Status

Work in progress — first commit scaffolds the taskgraph structure (config, docker images, parameters, test params). Remaining kinds (lint, test, frontend, build, deploy, hook, release) to follow.

Dependencies

Requires fxci-config mozilla-releng/fxci-config#882 to add `github-taskgraph: true` for the `code-review` project.

Merge order

1. Apply Enable github-taskgraph for code-review project mozilla-releng/fxci-config#882 to staging (`apply-staging` comment)
2. Verify this PR on staging
3. Merge Enable github-taskgraph for code-review project mozilla-releng/fxci-config#882 (auto-deploys to production)
4. Verify this PR on production
5. Merge this PR

Verification

TBD — staging validation after all commits are in place.