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/services/api/codexShim.ts

@@ -563,7 +563,7 @@ export async function performCodexRequest(options: {
     throw APIError.generate(
       response.status, errorResponse,
       `Codex API error ${response.status}: ${errorBody}`,
-      response.headers as unknown as Record<string, string>,
+      response.headers as unknown as Headers,
     )
   }
 
@@ -646,7 +646,7 @@ export async function collectCodexCompletedResponse(
     if (event.event === 'response.failed') {
       const msg = event.data?.response?.error?.message ??
         event.data?.error?.message ?? 'Codex response failed'
-      throw APIError.generate(500, undefined, msg, {} as Record<string, string>)
+      throw APIError.generate(500, undefined, msg, new Headers())
     }
 
     if (
@@ -661,7 +661,7 @@ export async function collectCodexCompletedResponse(
   if (!completedResponse) {
     throw APIError.generate(
       500, undefined, 'Codex response ended without a completed payload',
-      {} as Record<string, string>,
+      new Headers(),
     )
   }
 
@@ -820,7 +820,7 @@ export async function* codexStreamToAnthropic(
     if (event.event === 'response.failed') {
       const msg = payload?.response?.error?.message ??
         payload?.error?.message ?? 'Codex response failed'
-      throw APIError.generate(500, undefined, msg, {} as Record<string, string>)
+      throw APIError.generate(500, undefined, msg, new Headers())
     }
   }