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: each check it made against a running server
is now a handler test in internal/handlers. 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

internal/config/config.go

@@ -62,7 +62,7 @@ const (
 )
 
 // placeholderSigningKey is the dummy signing_key shipped in
-// config.example.yml. It is 45 characters, so it passes the length
+// configs/config.example.yml. It is 45 characters, so it passes the length
 // check, but it is public in this repository and must be rejected at
 // startup so no deployment ever signs URLs with it.
 const placeholderSigningKey = "CHANGE_ME_generate_with_openssl_rand_base64_32"
@@ -88,7 +88,7 @@ var (
 	errMustBeAtLeastOne  = errors.New("must be at least 1")
 	errValueTooShort     = errors.New("value too short")
 	errPlaceholderKey    = errors.New(
-		"is the placeholder from config.example.yml; " +
+		"is the placeholder from configs/config.example.yml; " +
 			"generate a real key with: openssl rand -base64 32")
 	errMustBeSetTogether = errors.New("must be set together")
 	errMustNotBeNegative = errors.New("must not be negative")
@@ -553,8 +553,8 @@ func (c *Config) ensureStateDirWritable() error {
 }
 
 // validateSigningKey checks that the signing key is present, long
-// enough, and not the public placeholder from config.example.yml. The
-// key value itself is never echoed in error messages.
+// enough, and not the public placeholder from configs/config.example.yml.
+// The key value itself is never echoed in error messages.
 func (c *Config) validateSigningKey() error {
 	if c.SigningKey == "" {
 		return fmt.Errorf("%s: %w", settingName(keySigningKey), errValueRequired)