Add code mode for composing session tools with JavaScript

[lukebuehler]

Code mode lets the model compose its existing session tools in a single JavaScript call: run independent work in parallel, loop over results, pass data between tools, and return only the useful output.

• Add the QuickJS-based codemode crate and code_execute, with async tool calls, execution limits, cancellation, and reports of partial outcomes. A dedicated Temporal workflow routes effects through the session's existing tool machinery without replaying the whole script.
• Support ordinary tools, native and discoverable MCP tools, environment jobs, sub-agents, and promise controls. Expose available output JSON Schemas alongside tool descriptions.
• Add shared content references and ordinary blob tools, reference-based file writes, environment file references, and persisted web-fetch bodies. Provide text(), media(), and file() helpers; expose UTF-8 process output as text while retaining binary output.
• Add the Code mode feature and advanced limits to the UI, distinct transcript styling, generated API/workflow contracts, and a dedicated user guide.

Validation:

• Workspace Clippy with all targets and warnings denied.
• Unit suites for codemode, harness, temporal-workflow, and tools.
• Full npm run check, including generated-artifact checks, typechecking, consumer tests, and production/demo builds.
• Documentation tests, Astro diagnostics, build, and link validation.
• Earlier implementation validation included native and Temporal live suites, actual-model execution, MCP discovery, jobs/sub-agents, media/file output, and worker-loss recovery; those credentialed suites were not rerun when opening this PR.