Add ampcode (Amp CLI) — DRAFT, proprietary + unscannable - #510

The Amp CLI, from the ampcode/homebrew-tap Homebrew formula, via
`pkgmgr import homebrew`.

Draft on purpose. This package is honest about two things that a
reviewer must decide, not two things the tooling failed to work out.

BOTH TODOs ARE RESOLVED IN THE FILE, not left open:

- CPE: NVD keywordSearch=ampcode returns ZERO results. No CPE identity
  exists for Amp, so there is nothing to attach — the package is
  unscannable by construction, not by omission. `sourcegraph` returns
  462 CPEs but every one is `sourcegraph:sourcegraph`, the code-search
  product, a DIFFERENT product; attaching it would import another
  product's CVEs as false positives, so it is deliberately not done.
- TERMS: ampcode.com/terms grants a "limited, revocable, non-exclusive,
  non-transferable, non-sublicensable" licence. Redistribution is NOT
  permitted. The package therefore fetches DIRECT from
  static.ampcode.com and is never mirrored to gs:// — the same shape
  chromium-bin uses against Google's and Playwright's CDNs.

No source_provenance is emitted. A vendor CDN URL yields none, and
fabricating a plausible-looking one is how chromium-bin came to name
microsoft/playwright for an artifact served out of Google's bucket.
Omission is honest; a wrong owner/repo is worse than nothing.

Requires stdlib 0.0.18 for the `redistributable` attr class
(minimal#861), hence the minimum_version bump — that is a repo-wide
change and the main reason this is a separate PR. Note nothing in
minimal READS the attr yet, so today it records the constraint rather
than enforcing it.

Two operational facts worth knowing before merging:
- The artifacts are ~111 MB each, ~222 MB per version across both
  arches, and the version is a unix timestamp that bumps on essentially
  every release (it moved twice while this was being prepared).
- runtime_deps is a discovery-mode guess of [glibc]. A 111 MB binary is
  likely a Node SEA or Bun-compiled blob; only a --from-build DT_NEEDED
  scan will tell the truth.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

The previous revision said nothing in minimal reads `redistributable`,
so the attr recorded a constraint rather than applying one. That was
wrong, and wrong in the direction that matters: a reviewer would have
concluded the proprietary binary could still reach the shared cache.

build-servers#180 (merged 2026-07-21) enforces it. Specs with
`attrs.redistributable = false` are excluded from cache upload, exempt
from the cache-audit completeness invariant, and skipped in the closure
seal — unconditionally, so post-purge seals record absence rather than a
seal-blocking hole. Consumers build locally via the planner's index-miss
fallback.

minimal#861 DECLARES the attr; the buildbot is what ACTS on it. Reading
only the stdlib side is what produced the wrong conclusion.

So two independent mechanisms now cover the licence constraint, and the
file says so: the Source urls fetch direct from static.ampcode.com (the
artifact is never mirrored to gs:// at all), and the attr keeps it out
of the shared cache if it ever were.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>