fix: Windows upgrade + uninstall route to upstream opencode package

## Summary

Multiple CLI paths reference the upstream `opencode` / `opencode-ai` package names instead of altimate-code's actual package identifiers, causing Windows users to hit the wrong product during upgrade and uninstall flows.

## Evidence

Production telemetry (last 3 days) shows Windows users receiving auto-upgrade attempts that target `1.14.19` — a version of the **upstream** `sst/opencode` package, not altimate-code. The stuck user ID `12c7c779-a669-4342-ad8d-ef518e640914` hit "not running from an elevated command shell" five times in a row trying to upgrade to the wrong product.

```
upgrade_attempted from=0.6.0 to=1.14.19 method=other status=error
error="not running from an elevated command shell" × 5
```

## Root causes

1. **`packages/opencode/src/installation/index.ts`** — `method()` auto-detected choco/scoop by looking for the upstream `opencode` package in those registries; `latest()` queried the upstream chocolatey feed / scoop bucket for version; `upgrade()` ran `choco upgrade opencode` / `scoop install opencode@...`.
2. **`packages/opencode/src/cli/cmd/upgrade.ts`** — `--method` option still advertised `choco` and `scoop` as valid choices even after the fix made them fail.
3. **`packages/opencode/src/cli/cmd/uninstall.ts`** — Same class of bug: `npm uninstall -g opencode-ai`, `brew uninstall opencode`, `choco uninstall opencode` — would either fail silently or uninstall the wrong product.

altimate-code is **not** distributed via chocolatey or scoop (verified: `publish.ts` only publishes to npm / Docker / AUR / Homebrew).

## Observability gap

The existing `upgrade_attempted` telemetry captured upgrade outcomes but not proactive notifications, so we couldn't compute a "user was told an upgrade exists → user upgraded" funnel. Without that signal, we can't distinguish "nobody knows 0.6.0 exists" from "people saw it and ignored it."

## Acceptance criteria

- [ ] Windows users running `altimate upgrade` via choco or scoop get a clear error pointing to npm / Homebrew / `https://altimate.ai/install` — never install the upstream product.
- [ ] `altimate upgrade --help` no longer offers `choco` / `scoop` as `--method` values.
- [ ] `altimate uninstall` uses altimate-code's real package identifiers (`@altimateai/altimate-code` for npm-family, `altimate-code` for brew).
- [ ] Auto-detection (`Installation.method()`) never returns `"choco"` or `"scoop"` — matching upstream `opencode` installed alongside would misroute upgrades.
- [ ] New `upgrade_available` telemetry event fires when users are proactively shown an "upgrade available" notification (dedup per correlation_id + version), enabling funnel analysis.

## Out of scope

- Adding chocolatey / scoop publishing pipelines for altimate-code (possible future work; not required for this fix).