fix(rebase): migrate the replayed E2B branch onto the rebased parent

The linear replay carried old-lineage content over parent-owned files;
this checkpoint restores them and adapts the branch to the parent's
post-rebase seam:

- restore all pty/lsp/subprocess/code-runtime surfaces to the parent's
  exact content (this branch claims none of them) and drop the net-zero
  code-runtime-e2b/pty-e2b/lsp-e2b residue and its registrations
- widen serializeRemoteEnvironment to the seam's NodeJS.ProcessEnv
  tombstone contract: an explicit undefined removes an ambient entry
- migrate the two E2B fixture Agent stubs to the Inbox-model interface
  and Session.create
- re-apply the branch's gen-doc-graphs roles, THIRD_PARTY_NOTICES e2b
  row, and packages/README group row (trimmed to the doc budget);
  regenerate catalogs and re-record bilingual pairings

packages/e2b/subprocess-e2b/src/environment.ts

@@ -84,19 +84,21 @@ export function bootstrapEnvironment(raw: string): Record<string, string> {
 /**
  * Overlay explicit entries and serialize one validated E2B environment.
  * @param raw - The complete NUL-delimited remote environment.
- * @param explicit - Deliberate caller overrides applied after ambient scrubbing.
+ * @param explicit - Deliberate caller overrides applied after ambient scrubbing; an `undefined` tombstone removes an ambient entry.
  * @returns NUL-delimited `name=value` entries accepted by `env -i`.
  */
 export function serializeRemoteEnvironment(
   raw: string,
-  explicit: Readonly<Record<string, string>> | undefined,
+  explicit: Readonly<NodeJS.ProcessEnv> | undefined,
 ): string {
   const environment = scrubRemoteEnvironment(raw)
   for (const [name, value] of Object.entries(explicit ?? {})) {
-    if (name.length === 0 || name.includes('=') || name.includes('\0') || value.includes('\0')) {
+    if (name.length === 0 || name.includes('=') || name.includes('\0') || value?.includes('\0') === true) {
       throw new Error('subprocess-e2b: environment entries require non-empty NUL-free names without = and NUL-free values')
     }
-    environment.set(name, value)
+    // An explicit undefined is the seam's tombstone: remove the ambient entry.
+    if (value === undefined) environment.delete(name)
+    else environment.set(name, value)
   }
   return [...environment].map(([name, value]) => `${name}=${value}\0`).join('')
 }