fix(packaging): ship bundled plugin.yaml manifests in wheel and sdist

The v0.15.0 PyPI wheel shipped every plugin's Python code but none of its
plugin.yaml manifests, so plugin discovery (hermes_cli/plugins.py) found zero
plugins and ALL gateway platforms failed with "No adapter available for
<platform>" (discord, slack, mattermost, ...). Same gap also dropped the
web-search provider manifests (#28149).

Declare manifest coverage in both packaging channels:
- wheel: [tool.setuptools.package-data] plugins += **/plugin.yaml, **/plugin.yml
- sdist: MANIFEST.in recursive-include plugins plugin.yaml plugin.yml
  (Homebrew and other downstream packagers build from the sdist)

Verified by building the wheel before/after: plugin.yaml count went 0 -> 69,
discord's manifest now ships. Adds a regression test asserting both channels
cover manifests.

Fixes #34034

Co-authored-by: outsourc-e <201563152+outsourc-e@users.noreply.github.com>
Co-authored-by: Dhruvil Parikh <41384593+dparikh79@users.noreply.github.com>
Co-authored-by: ousiaresearch <261687298+ousiaresearch@users.noreply.github.com>
Co-authored-by: libre-7 <6366424+libre-7@users.noreply.github.com>

pyproject.toml

@@ -226,6 +226,14 @@ plugins = [
   "*/dashboard/manifest.json",
   "*/dashboard/dist/*",
   "*/dashboard/dist/**/*",
+  # Plugin discovery (hermes_cli/plugins.py) reads a plugin.yaml/plugin.yml
+  # manifest from each bundled plugin directory to register it. Wheels only
+  # carry files declared here, so without this glob the wheel ships every
+  # plugin's Python code but none of its manifests — the scan finds zero
+  # plugins and all gateway platforms fail with "No adapter available for
+  # <platform>" (#34034), web-search providers go missing (#28149), etc.
+  "**/plugin.yaml",
+  "**/plugin.yml",
 ]
 
 [tool.setuptools.packages.find]