Move the example config to configs/, delete scripts/ and CONVENTIONS.md (closes #97)

config.example.yml moves unchanged to configs/config.example.yml, the
directory REPO_POLICIES.md names for configuration examples. README.md,
the comments in internal/config/config.go and the startup error for the
placeholder signing key name the new path. scripts/manual-test.sh and
its directory are deleted. The handler tests in internal/handlers cover
every check it made except two: fetching a real image from the internet,
and a URL made on the generator page with a ttl answering 410 once the
ttl has passed (#199).
CONVENTIONS.md, a reformatted copy of the Go HTTP server conventions, is
deleted, as REPO_POLICIES.md links the canonical document.

Model: opus-5-5

README.md

@@ -18,7 +18,7 @@ make build
 # run with a config file: copy the example and set a real signing key
 # (the example placeholder is refused at startup), e.g. with
 # openssl rand -base64 32
-cp config.example.yml config.yml
+cp configs/config.example.yml config.yml
 $EDITOR config.yml   # replace the signing_key placeholder
 ./bin/pixad --config config.yml
 
@@ -514,7 +514,7 @@ Key settings in more detail:
   the container unhealthy, and upaas marks a deploy failed when its container
   is unhealthy. The login and URL generator pages and `/metrics` keep working
 
-See `config.example.yml` for all options with defaults.
+See `configs/config.example.yml` for all options with defaults.
 
 ### Architecture