Documentation covers standing up the ha-terminal profile on the HA SSH addon

## Summary
A how-to for the addon side plus architecture rationale for the host-role model, so the `ha-terminal` profile is reproducible and its design decisions are recorded.

## Motivation
The profile (#217) needs a setup path — the addon `packages`/`init_commands` config, the chezmoi invocation that passes the role, and the amd64 assumption — and the rationale recorded where the rest of the architecture lives (Diátaxis: how-to + explanation).

## Scope
- `docs/how-to/`: configure the HA "Advanced SSH & Web Terminal" addon (`packages`, `init_commands`, passing the role via `CHEZMOI_ROLE`) and bootstrap the profile end to end.
- `docs/explanation/architecture.md`: cross-link the "Host roles" section (from #218); record the explicit-role and no-secrets decisions and the amd64-only assumption (note arm64 as future work).
- `README.md`: note the second supported platform if warranted.

## Acceptance criteria
- [ ] A reader can stand up the profile on a fresh addon from the how-to alone.
- [ ] Architecture explains why explicit role over detection, why no long-lived secrets, and why amd64-only for now.

## Additional context
Parent: #217. Depends on #218, #219, #220.

The role is passed via `CHEZMOI_ROLE` only. The `--promptString role=` path was deliberately not wired in #218/#417: it would add an interactive prompt to a plain `chezmoi init`, and the addon's unattended `init_commands` re-runs want the env var anyway. Standardize the how-to on `CHEZMOI_ROLE` — don't reintroduce the prompt.