chore: update Homebrew formula to v0.15.0

[subinium]

Summary

Update only homebrew/agf.rb to the published AGF 0.15.0 release and its four
macOS/Linux architecture-specific SHA-256 checksums. No application source,
lockfile or runtime behavior changes.

Verification

• Release workflow 33982590150 completed successfully (17/17 jobs) on
  7a426265b49cd9757cb111902abc8fdfa34536e9.
• All five downloaded public release archives match checksums.txt.
• The published macOS ARM64 and Linux x86_64 archives are byte-identical to the
  workflow artifacts already executed independently.
• macOS ARM64 binary: isolated JSON and read-only MCP smoke passed.
• Linux x86_64 binary: version and structured capabilities passed in a
  network-disabled, read-only Ubuntu 24.04 container.
• Exact crates.io 0.15.0 checksum and clean release-commit provenance verified;
  the release workflow installed that registry version and passed JSON/MCP smoke.
• Ruby formula syntax is valid. The full 18-stage local gate passed again on an
  unchanged formula-only source snapshot: 266 debug, 266 release and 255 minimal
  tests, plus all-target warning-denying Clippy, MSRV 1.88, audit, docs, package
  and installed-binary JSON/MCP checks.
• Public tap updated at subinium/homebrew-tap@ac4e3c272573e5151c6c294a65e3f8e29e2ae5dd;
  the fetched remote formula matches this file exactly.
• Actual Homebrew --build-from-source install, formula brew test and the
  installed binary's JSON/MCP smoke passed. A temporary link required by
  brew test was removed afterwards; the previous PATH and Homebrew developer
  mode were preserved. No real provider session was executed.

The release tag remains on the original audited source commit. This follow-up
does not rebuild or replace already published archives.