rewrite README, add MIT license, rename vendored patch doc

- self-contained README (removed references to deleted ARCHITECTURE/GOAL docs)
- add LICENSE (MIT)
- rename third_party/certspotter/CTCLOUD_PATCH.md -> BUCKETEER_PATCH.md and
  update the in-code references

third_party/certspotter/BUCKETEER_PATCH.md

@@ -0,0 +1,53 @@
+# Bucketeer patch to upstream CertSpotter
+
+This directory is a copy of upstream CertSpotter
+(https://github.com/SSLMate/certspotter), pulled in as a local module and
+wired up with a `replace` directive in the root `go.mod`.
+
+It is intentionally a copy (not a git submodule) so the patched tree lives in
+this repository and is compiled by the Docker build with no network access to a
+separate fork. To rebase on a newer upstream release:
+
+1. Replace this directory with the new upstream tree.
+2. Re-apply the two hunks below.
+
+## Changes (only two hunks)
+
+### `monitor/config.go`
+
+Adds an exported `IdentifierSink` interface and a `Config.IdentifierSink`
+field:
+
+```go
+type IdentifierSink interface {
+	HandleIdentifiers(ctx context.Context, identifiers *certspotter.Identifiers)
+}
+```
+
+### `monitor/process.go`
+
+In `processCertificate`, immediately after `certInfo.ParseIdentifiers()`, hand
+the parsed identifiers to the sink and return, bypassing the watchlist,
+certificate-chain retrieval and notification paths:
+
+```go
+if config.IdentifierSink != nil {
+	config.IdentifierSink.HandleIdentifiers(ctx, identifiers)
+	return nil
+}
+```
+
+## Why
+
+The sink gives Bucketeer parsed DNS identifiers via a direct in-process Go call:
+
+```text
+CT log entry -> CertSpotter parse -> IdentifierSink.HandleIdentifiers -> classifier
+```
+
+No stdout, no JSON, no files, no hooks, no subprocess. Setting the sink also
+skips issuer-chain fetching and the watchlist machinery, which is exactly the
+I/O and serialization overhead the design forbids.
+
+Upstream is otherwise unmodified. License and notices are preserved in
+`LICENSE`.