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/permissions/yoloClassifier.ts

@@ -780,7 +780,6 @@ async function classifyYoloActionXml(
         model,
         max_tokens: (mode === 'fast' ? 256 : 64) + thinkingPadding,
         system: systemBlocks,
-        skipSystemPromptPrefix: true,
         temperature: 0,
         thinking: disableThinking,
         messages: [
@@ -867,7 +866,6 @@ async function classifyYoloActionXml(
       model,
       max_tokens: 4096 + thinkingPadding,
       system: systemBlocks,
-      skipSystemPromptPrefix: true,
       temperature: 0,
       thinking: disableThinking,
       messages: [
@@ -1141,7 +1139,6 @@ export async function classifyYoloAction(
           cache_control: getCacheControl({ querySource: 'auto_mode' }),
         },
       ],
-      skipSystemPromptPrefix: true,
       temperature: 0,
       thinking: disableThinking,
       messages: [