Add a usage reader for Amp (Sourcegraph)

[toshipepe]

Tokimeter reads usage metadata coding agents already write locally and folds every tool into one meter. Amp is a popular agent we do not cover yet.

What's needed:

1. Locate Amp's local per-request/session usage store and document its format with a small scrubbed fixture.
2. Add a pure event mapper and local collector using the opencode reader as the pattern.
3. Gate collection on --tool amp.
4. Add fixtures, parser tests, and a no-content/no-PII assertion.
5. Add Amp to the README support table with an honest verification level.

Ground rules: metadata only; never prompt or response content; no extra model calls; exact tokens where available and ~$ for estimates. See CONTRIBUTING.md.

If Amp exposes no readable local usage data, a documented finding with evidence is a useful outcome too.