ux: warn at startup if multiple altimate binaries are on $PATH

Found during v0.7.1 release review (Chaos Gremlin / Supply Chain Auditor).

After the v0.7.1 rename, a user who installed v0.7.0 via npm and switches to brew (or vice versa) ends up with two `altimate` binaries on PATH at different versions from different vendors. We have no detection or warning. Same for curl-installed (`~/.altimate/bin/altimate`) coexisting with an npm install (`/bin/altimate`).

## Acceptance

On startup, run `which -a altimate` (or equivalent on Windows) and log a one-line warning if more than one path resolves. Suggest uninstalling the duplicate. Suppressed in test/CI environments via existing telemetry-disabled env vars.

## Why this isn't in v0.7.1

Not a regression in PR #820 — the multi-install scenario was always possible. Surfacing the warning is a small but real UX feature, deserves its own PR with proper test coverage.