feat(cli): add runtime-backed shell mode and streamline CLI docs

[limityan]

Summary

• Add an OpenCode-compatible Shell composer entered by typing ! into an empty prompt. The marker is presentation-only; / remains literal shell input, Escape and empty Backspace exit, chat/shell histories stay separate, and unsupported image or structured-reference payloads fail before submission.
• Route user-authored commands through a narrow AgentUserShellCommandPort in the Rust Runtime SDK. Core reuses the existing ExecCommand ToolPipeline, permission policy, local/remote workspace ports, cancellation, audit, persistence, and Agentic events; the CLI never spawns the command itself.
• Extend the private Shared TUI IPC with the closed run_user_shell_command operation and protocol version 12, preserving current-controller, idle, timeout, disconnect-cancellation, and outcome-unknown rules.
• Streamline the CLI user guide and ownership documentation, and condense the CLI architecture document around durable current-state boundaries.

Architecture and behavior

• No /shell alias or new slash command is introduced; the entry and exit behavior follows OpenCode.
• Shell execution is a standard model-visible UserDialog plus ModelRound tool call/result, so Desktop restore and later model context observe the same persisted facts.
• Explicit user input may auto-approve interactive ask, while static global/project/profile deny rules remain authoritative.
• The ToolPipeline derives a child cancellation token from the accepted turn token before validation, so cancellation is latched before permissions or process side effects can begin.
• Shared IPC remains a first-party local TUI adapter, not a public SDK, remote protocol, or generic Tool/process API. Protocol 12 requires a matching Shared Runtime/TUI binary pair.

Validation

• cargo check -p bitfun-cli -p bitfun-core -p bitfun-agent-runtime-ipc -p bitfun-agent-runtime -p bitfun-runtime-ports
• cargo test -p bitfun-core --lib --no-fail-fast — 1768 passed, 1 ignored
• cargo test -p bitfun-core user_shell_command -- --nocapture — 5 passed
• cargo test -p bitfun-agent-runtime -p bitfun-agent-runtime-ipc --no-fail-fast — passed; IPC 53/53
• Focused CLI Shell-mode and programmatic-prefill tests — 3/3 passed
• pnpm run check:core-boundaries and pnpm run check:core-boundaries:test — passed; 32/32 boundary tests
• git diff --check and local Markdown-link validation — passed
• Independent adversarial review after implementation — no remaining P0-P3 findings

Full Windows CLI tests reached 600/603. The three failures are unchanged Dispatch workspace/platform tests outside this diff: two require detached workers that currently support only Linux/macOS, and one passes a \\?\\C: worktree path that Git for Windows rejects. cargo check --workspace could not complete in this local environment because src/mobile-web/dist was absent and the sherpa-onnx prebuilt download ended early; the affected crates above pass.

Out of scope

• Relay behavior, GUI Shell input, public SDK/App Server protocol, generic Tool execution APIs, CI workflow changes, and the unrelated Windows Dispatch failures.
• No superpowers documentation is included.

Implementation was AI-assisted; testing level is extensive local verification as listed above.