feat: add download, btw, and subagent status

[zha-chicken]

Issue for this PR

N/A — this is a feature PR and has no linked issue.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds three opt-in TUI workflows:

• A streamed download tool for large HTTP(S) files. It reports bounded progress inside one tool call, writes to an adjacent partial file, optionally verifies SHA-256, and atomically moves the completed file into place.
• /btw, a tool-free side conversation using a sanitized snapshot of the current session. It strips tool history and hidden reasoning and does not persist into or interrupt the main conversation.
• Active Subagent cards below LSP, showing the agent name and elapsed time. Cards disappear when the child session becomes idle.

The OpenAPI/TypeScript SDK output and user documentation are updated for the new endpoint, command, and tool.

How did you verify your code works?

• Core, TUI, and generated SDK typechecks pass.
• Download tests: 2 passed.
• /btw context-isolation and retry tests: 3 passed.
• Legacy tool registry tests: 15 passed.
• Affected core regression groups: 116 passed.
• Full TUI suite: 201 passed, 1 skipped.
• Changed-file lint reports 0 errors, and git diff --check passes.

The full OpenCode package typecheck still reproduces the existing src/bus/global.ts TS2416 EventEmitter incompatibility from the checked-out dev baseline.

Screenshots / recordings

Not included yet; this PR is still a draft. The visible changes are the terminal download card, /btw dialog, and active Subagents sidebar cards.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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