Correct release-blocking README and startup-warning inaccuracies (closes #151)

Publishing this README would have shipped false statements about the
product. Corrects the eight items on the issue plus everything a full
sweep turned up: the Slack circuit-breaker scope, a nonexistent WAL, the
wrong config key for slack targets, six undocumented routes, the
conditional /metrics registration, wrong retention bands, wrong shutdown
mechanism, and a Quick Start that led a new contributor into a red
build.

The lockout warning now fires whenever TRUSTED_PROXIES is empty rather
than only in production, since the variable it was gated on defaults to
dev. Rate-limit keying, the limits and the TRUSTED_PROXIES default are
untouched — those belong to #150.

What /s/* actually serves was settled empirically rather than by
reading: all five of GET/HEAD/POST/PUT/DELETE return 200, pinned by
TestStaticServesEveryMethod. Restricting it is filed separately.

Independently reviewed after three prior rounds. The reviewer
re-derived all fifteen claim-table rows against the code, including
every row a previous revision had marked "correct, left alone" and got
wrong, and found zero false; then verified every route method-by-method,
all twelve environment variables, all nine entity tables, and the
package tree against git ls-files. The Quick Start was confirmed by
running it in a fresh clone.

internal/config/config_test.go

@@ -628,10 +628,12 @@ func testTrustedProxiesSuccess(
 }
 
 // TestSharedRateLimitBucketWarning covers the startup warning that
-// tells an operator their production deployment shares one rate-limit
-// bucket between every client, which makes the admin login remotely
-// deniable. It must fire when TRUSTED_PROXIES is empty in production
-// and stay quiet otherwise.
+// tells an operator a deployment behind a reverse proxy shares one
+// rate-limit bucket between every client, which makes the admin login
+// remotely deniable. It must fire whenever TRUSTED_PROXIES is empty,
+// in any environment: WEBHOOKER_ENVIRONMENT defaults to dev, so gating
+// on it would silence the warning for exactly the operator who never
+// configured the deployment. It stays quiet once proxies are named.
 func TestSharedRateLimitBucketWarning(t *testing.T) {
 	tests := []struct {
 		name           string
@@ -651,12 +653,19 @@ func TestSharedRateLimitBucketWarning(t *testing.T) {
 			expectWarning:  false,
 		},
 		{
-			// Development is not required to run behind a
-			// reverse proxy, so the shared bucket the warning
-			// describes is not the expected shape there.
-			name:          "dev without trusted proxies is quiet",
+			// The default environment. An internet-exposed
+			// deployment whose operator never set
+			// WEBHOOKER_ENVIRONMENT lands here and has exactly
+			// the exposure the warning announces.
+			name:          "dev without trusted proxies warns",
 			environment:   config.EnvironmentDev,
-			expectWarning: false,
+			expectWarning: true,
+		},
+		{
+			name:           "dev with trusted proxies is quiet",
+			environment:    config.EnvironmentDev,
+			trustedProxies: cidrPrivateV4,
+			expectWarning:  false,
 		},
 	}
 
@@ -697,8 +706,14 @@ func TestSharedRateLimitBucketWarning(t *testing.T) {
 
 			assert.Contains(t, logged, `"level":"WARN"`)
 			assert.Contains(t, logged, "TRUSTED_PROXIES")
-			assert.Contains(t, logged, "shares one bucket")
-			assert.Contains(t, logged, "deny the admin login")
+			assert.Contains(t, logged, "share one bucket")
+			assert.Contains(t, logged, "denying the admin login")
+			// The text must stay accurate for a developer with
+			// nothing in front of the process, where an empty
+			// list costs nothing.
+			assert.Contains(
+				t, logged, "nothing proxying to this process",
+			)
 		})
 	}
 }