feat: collect workspace logs in support bundles (#26694)

Add workspace-side file collection to `coder support bundle` via
repeatable --workspace-file flags. The agent resolves the requested
paths or globs inside the remote workspace and streams back a tar with
a manifest and the collected files; nothing is read from the machine
running the command.

- Add POST /api/v0/bundle-files to the agent's agentfiles package.
- Expand env vars in the agent's environment; paths must then be
  absolute or start with ~/ (the agent user's home directory).
- Support ** globs and tail oversized files.
- Record requested patterns, per-path errors, truncation, and the
  applied limits in a manifest.
- Unpack the archive into the bundle under agent/workspace_files/,
  recording dropped entries in collection_errors.txt.
- Write a manifest-only archive marking collection as unsupported for
  agents that predate the endpoint.
- Bound collection: 64 KB request body, 10000 files, 10 MiB per file,
  100 MiB total including archive overhead, 110 MiB client-side read
  cap, 5 minute timeout.

Closes #26020

testutil/archive.go

@@ -2,7 +2,10 @@ package testutil
 
 import (
 	"archive/tar"
+	"archive/zip"
 	"bytes"
+	"errors"
+	"io"
 	"path/filepath"
 	"testing"
 
@@ -59,3 +62,39 @@ func CreateZip(t testing.TB, files map[string]string) []byte {
 	require.NoError(t, err)
 	return za
 }
+
+// Reads every entry of the in-memory tar into a map keyed by entry name.
+func ReadTar(t testing.TB, data []byte) map[string][]byte {
+	t.Helper()
+
+	entries := make(map[string][]byte)
+	tr := tar.NewReader(bytes.NewReader(data))
+	for {
+		hdr, err := tr.Next()
+		if errors.Is(err, io.EOF) {
+			return entries
+		}
+		require.NoError(t, err)
+		content, err := io.ReadAll(tr)
+		require.NoError(t, err)
+		entries[hdr.Name] = content
+	}
+}
+
+// Reads every entry of the in-memory zip into a map keyed by entry name.
+func ReadZip(t testing.TB, data []byte) map[string][]byte {
+	t.Helper()
+
+	zr, err := zip.NewReader(bytes.NewReader(data), int64(len(data)))
+	require.NoError(t, err)
+	entries := make(map[string][]byte, len(zr.File))
+	for _, file := range zr.File {
+		rc, err := file.Open()
+		require.NoError(t, err)
+		content, err := io.ReadAll(rc)
+		_ = rc.Close()
+		require.NoError(t, err)
+		entries[file.Name] = content
+	}
+	return entries
+}