Add a new observability backend to otel-as-code

The machine-readable vendor list has a single source (backends.txt). Adding a backend is: register it there, add its per-vendor content, then a validated snapshot. Ask for the vendor name and its Terraform provider source if not given.

1. Register the vendor (single source of truth)

Add the vendor name (one per line) to backends.txt at the repo root. This is the ONLY place the list is enumerated — the CI validate-terraform loop, tests/check-snapshots.sh, the session-summary hook, and /otel-backend's validation all read it.

2. terraform-patterns skill

Add a ## <Vendor> section to skills/terraform-patterns/SKILL.md: provider source + version, auth variables, required resources (dashboard / alerts / SLO), key gotchas, and OTel-specific query examples (PromQL/NRQL/native).

3. terraform-gen subagent

Add a ## <Vendor> main.tf block to agents/terraform-gen.md describing the resources to emit, referencing the skill section.

4. Golden snapshot (validated)

Create tests/snapshots/<vendor>/main.tf.snap:

• Make it SELF-CONTAINED — declare its own variable blocks with defaults — so CI can terraform validate main.tf standalone (the job copies only main.tf.snap).
• For an unfamiliar provider, introspect the real schema instead of guessing: after terraform init, run terraform providers schema -json to get exact resource names and required fields.
• Validate: copy to a temp dir, terraform init -backend=false + terraform validate (download a pinned terraform binary if absent). Iterate until it passes; then terraform fmt.

5. Human-facing docs

Update the backend list in README.md and the design docs. No CI or /otel-backend command edits are needed for the list itself — those read backends.txt.

Verify

Run /validate-plugin and confirm the new snapshot validates and the CI vendor loop stays green.