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/sideQuery.ts

@@ -51,7 +51,7 @@ export type SideQueryOptions = {
   maxRetries?: number
   /** Abort signal */
   signal?: AbortSignal
-  /** Skip CLI system prompt prefix (keeps attribution header for OAuth). For internal classifiers that provide their own prompt. */
+  /** Skip CLI system prompt prefix (keeps attribution header for OAuth). Default true — side queries are internal classifiers with their own prompt. Set false only for queries that need the full "You are Claude Code…" prefix. */
   skipSystemPromptPrefix?: boolean
   /** Temperature override */
   temperature?: number
@@ -115,7 +115,7 @@ export async function sideQuery(opts: SideQueryOptions): Promise<BetaMessage> {
     max_tokens = 1024,
     maxRetries = 2,
     signal,
-    skipSystemPromptPrefix,
+    skipSystemPromptPrefix = true,
     temperature,
     thinking,
     stop_sequences,