chore: delete scripts/ and cut requirements.txt down to its real consumers

scripts/ held the pre-marketplace MCP servers (gmail, gcal, gmaps, ssh, weather,
google_trends, whatsapp, serpapi_flights). Nothing referenced them any more:
connectors are admin-curated and installed into connectors/ from the
marketplace, and ci/package.sh never shipped scripts/ in the first place — so on
every installed box requirements.txt was pulling google-auth, googlemaps,
paramiko, trendspyg and friends for files that did not exist there.

requirements.txt now states what it is actually for: the two TTS plugins, which
spawn a bare `python3` on an embedded server script and so have no dependency
reconciler of their own. A connector's deps stay with the connector —
`ensure_installed` puts them in .pydeps/node_modules inside the user's
container, `ensure_installed_host` beside the files for a global one.

The venv itself stays load-bearing for those two plugins and for the host pip
that installs a global connector's deps, so the run/install/update scripts keep
creating it — but their "Python MCP servers will be unavailable" warning was
naming the one thing that no longer depends on it, and now says what really
breaks.

CONNECTOR_MANIFEST_GUIDE.md moves to the repo root: it was the one thing in
scripts/ still referenced (CLAUDE.md), and being under a gitignored directory it
had never been committed at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

agents/software-engineer/AGENT.md

@@ -116,7 +116,6 @@ When working on **Skald itself** (the project you are in), follow these addition
   - Agent prompts: `agents/`
   - Extracted crates: `crates/`
   - Web app (Lit components): `web/`
-  - Python MCP scripts: `scripts/`
   - Config: `config.yml`
   - Docs: `docs/`
   - Database: `database.db`