[rekram1-node]

Problem

Several core services (pty, plugin, question, and snapshot) carried redundant runtime checks, unnecessary mutable type wrappers, or duplicate path/ID branding on already-typed values.

Change

Tighten the domain types and runtime checks across those modules:

 packages/core/src/
 ├── tool/plugin/question.ts  # narrow Form.Value with Array.isArray instead of typeof === "object"
 ├── pty.ts                   # drop DeepMutable on read-only inputs and redundant typeof check
 ├── plugin.ts                # preserve branded Plugin.ID on Slot["plugin"] instead of re-branding
 └── snapshot.ts              # short-circuit empty file/diff lists and brand RelativePath once per diff

 Snapshot.files(input)
   changed = git.tree.files(...)
+  if changed is empty -> return []
   skipped = ignored(changed)
   return changed without skipped

 Snapshot.diff(input)
   diffs = git.tree.diff(...)
+  if diffs is empty -> return []
+  paths = diffs.map(d => RelativePath.make(d.file))
-  skipped = ignored(diffs.map(d => RelativePath.make(d.file)))
-  return diffs.filter(d => !skipped.has(RelativePath.make(d.file)))
+  skipped = ignored(paths)
+  return diffs.filter((_, i) => !skipped.has(paths[i]))

Out of scope

• Changing Form.validateAnswer (isStringArray is retained to guard untrusted plugin answers at runtime).
• Changing McpClient.ElicitationResult or TypeScript's Array.isArray narrowing behavior in the false branch of mcp/index.ts.

[opencode-agent]

The pty, plugin, snapshot and question changes are sound: type-check passes and the core tests for form, snapshot, plugin, question and pty pass. Public API unchanged.

One concern, inline: dropping isStringArray removes the check on multiselect items in Form.validateAnswer, which is the only runtime check on answers from plugins.