fix: disable experimental API betas by default, reduce side query token usage, standardize Headers type (#281)

* fix: disable experimental API betas by default to prevent 500 errors

Tool search (defer_loading), global cache scope, and context management
betas require internal Anthropic server-side support. External accounts
receive 500 Internal Server Error when these are sent.

Set CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS=true by default in the CLI
entrypoint. Users with internal access can opt back in with =false.

Also includes: cache key stability fixes (Sonnet 1M latch, system-before-
messages key ordering, resume fingerprint isMeta skip), sideQuery default
cleanup, and /dream command.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: standardize API headers to Headers type and enable tengu feature flags by default

* fix: address PR review — dream lock, MCP betas guard, redundant Partial

- Call recordConsolidation() programmatically in /dream instead of
  delegating to model prompt (unreliable)
- Add CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS guard to MCP entrypoint
  (was only in CLI entrypoint, causing 500s in MCP server mode)
- Remove redundant ? markers from SecretValueSource Partial<{}> type

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/utils/fingerprint.ts

@@ -16,7 +16,13 @@ export const FINGERPRINT_SALT = '59cf53e54c78'
 export function extractFirstMessageText(
   messages: (UserMessage | AssistantMessage)[],
 ): string {
-  const firstUserMessage = messages.find(msg => msg.type === 'user')
+  // Skip isMeta messages (system-injected attachments) so the fingerprint
+  // reflects the actual user input. On --resume, reorderAttachmentsForAPI
+  // can bubble meta messages before the real first user message, changing
+  // the fingerprint and breaking cache.
+  const firstUserMessage =
+    messages.find(msg => msg.type === 'user' && !msg.isMeta) ??
+    messages.find(msg => msg.type === 'user')
   if (!firstUserMessage) {
     return ''
   }