The README badges name the platform and the license, but not the hosts #547

The badge row carries Tests, Python, OS, License and Version. It does not say which
agent hosts this plugin runs on -- and there are now three, each with a checked-in
manifest in this repo:

A reader deciding whether this plugin is usable from Codex or Gemini CLI has to read
the tree to find out. Add three host badges to the row.

Two things the badge must not overclaim, both already this repo's own convention
(the Windows caveat directly under the badge row exists for exactly this reason):

• Claude Code and Codex are observed. Codex is registered in a live
  ~/.codex/config.toml with four hooks.state entries, so hooks demonstrably fire.
• Gemini CLI is not. Gemini CLI: no working credentials in this environment -- blocks driving any live session for #456's remaining scope #532 is open because headless OAuth returns invalid_grant
  and no GEMINI_API_KEY / GOOGLE_API_KEY is set. Every Gemini test in the suite
  (test_gemini_manifest_456, test_gemini_extension_533,
  test_gemini_project_dir_var_456, test_gemini_stdout_envelope_534) asserts
  manifest shape; none asserts a hook ever fired under Gemini CLI. The maintainer
  has ruled the badge ships anyway, so the honesty belongs in the caveat paragraph
  rather than in the badge's absence: extend it to say the Gemini CLI extension ships
  tested manifests but has not been observed running end to end.

Out of scope here, worth its own issue: the H1 and the image alt text still read
"for Claude Code" while three host badges sit beneath them.