feat: add boundary log forwarding from agent to coderd (#21345)

Add agent forwarding of boundary audit logs from workspaces to coderd
via agent API, and re-emission of boundary logs to coderd stderr. This
change adds a server to the workspace agent that always listens on a
unix socket for boundary to connect and send audit logs.

coderd log format example:
```
[API] 2025-12-23 18:31:46.755 [info] coderd.agentrpc: boundary_request owner=.. workspace_name=.. agent_name=.. decision=.. workspace_id=.. http_method=.. http_url=.. event_time=.. request_id=..
```

Corresponding boundary PR: https://github.com/coder/boundary/pull/124
RFC:
https://www.notion.so/coderhq/Agent-Boundary-Logs-2afd579be59280f29629fc9823ac41ba
https://github.com/coder/coder/issues/21280

agent/boundarylogproxy/codec/codec.go

@@ -0,0 +1,127 @@
+// Package codec implements the wire format for agent <-> boundary communication.
+//
+// Wire Format:
+//   - 8 bits: big-endian tag
+//   - 24 bits: big-endian length of the protobuf data (bit usage depends on tag)
+//   - length bytes: encoded protobuf data
+//
+// Note that while there are 24 bits available for the length, the actual maximum
+// length depends on the tag. For TagV1, only 15 bits are used (MaxMessageSizeV1).
+package codec
+
+import (
+	"encoding/binary"
+	"io"
+
+	"golang.org/x/xerrors"
+)
+
+type Tag uint8
+
+const (
+	// TagV1 identifies the first revision of the protocol. This version has a maximum
+	// data length of MaxMessageSizeV1.
+	TagV1 Tag = 1
+)
+
+const (
+	// DataLength is the number of bits used for the length of encoded protobuf data.
+	DataLength = 24
+
+	// tagLength is the number of bits used for the tag.
+	tagLength = 8
+
+	// MaxMessageSizeV1 is the maximum size of the encoded protobuf messages sent
+	// over the wire for the TagV1 tag. While the wire format allows 24 bits for
+	// length, TagV1 only uses 15 bits.
+	MaxMessageSizeV1 uint32 = 1 << 15
+)
+
+var (
+	// ErrMessageTooLarge is returned when the message exceeds the maximum size
+	// allowed for the tag.
+	ErrMessageTooLarge = xerrors.New("message too large")
+	// ErrUnsupportedTag is returned when an unrecognized tag is encountered.
+	ErrUnsupportedTag = xerrors.New("unsupported tag")
+)
+
+// WriteFrame writes a framed message with the given tag and data. The data
+// must not exceed 2^DataLength in length.
+func WriteFrame(w io.Writer, tag Tag, data []byte) error {
+	var maxSize uint32
+	switch tag {
+	case TagV1:
+		maxSize = MaxMessageSizeV1
+	default:
+		return xerrors.Errorf("%w: %d", ErrUnsupportedTag, tag)
+	}
+
+	if len(data) > int(maxSize) {
+		return xerrors.Errorf("%w for tag %d: %d > %d", ErrMessageTooLarge, tag, len(data), maxSize)
+	}
+
+	var header uint32
+	//nolint:gosec // The length check above ensures there's no overflow.
+	header |= uint32(len(data))
+	header |= uint32(tag) << DataLength
+
+	if err := binary.Write(w, binary.BigEndian, header); err != nil {
+		return xerrors.Errorf("write header error: %w", err)
+	}
+	if _, err := w.Write(data); err != nil {
+		return xerrors.Errorf("write data error: %w", err)
+	}
+
+	return nil
+}
+
+// ReadFrame reads a framed message, returning the decoded tag and data. If the
+// message size exceeds MaxMessageSizeV1, ErrMessageTooLarge is returned. The
+// provided buf is used if it has sufficient capacity; otherwise a new buffer is
+// allocated. To reuse the buffer across calls, pass in the returned data slice:
+//
+//	buf := make([]byte, initialSize)
+//	for {
+//	    _, buf, _ = ReadFrame(r, buf)
+//	}
+func ReadFrame(r io.Reader, buf []byte) (Tag, []byte, error) {
+	var header uint32
+	if err := binary.Read(r, binary.BigEndian, &header); err != nil {
+		return 0, nil, xerrors.Errorf("read header error: %w", err)
+	}
+
+	const lengthMask = (1 << DataLength) - 1
+	length := header & lengthMask
+	const tagMask = (1 << tagLength) - 1 // 0xFF
+	shifted := (header >> DataLength) & tagMask
+	if shifted > tagMask {
+		// This is really only here to satisfy the gosec linter. We know from above that
+		// shifted <= tagMask.
+		return 0, nil, xerrors.Errorf("invalid tag: %d", shifted)
+	}
+	tag := Tag(shifted)
+
+	var maxSize uint32
+	switch tag {
+	case TagV1:
+		maxSize = MaxMessageSizeV1
+	default:
+		return 0, nil, xerrors.Errorf("%w: %d", ErrUnsupportedTag, tag)
+	}
+
+	if length > maxSize {
+		return 0, nil, ErrMessageTooLarge
+	}
+
+	if cap(buf) < int(length) {
+		buf = make([]byte, length)
+	} else {
+		buf = buf[:length:cap(buf)]
+	}
+
+	if _, err := io.ReadFull(r, buf[:length]); err != nil {
+		return 0, nil, xerrors.Errorf("read full error: %w", err)
+	}
+
+	return tag, buf[:length], nil
+}