feat(request-help): add BSK_REQUEST_HELP=off to disable blocking help requests

[BB-fat]

Motivation

bsk request-help is a blocking human-in-the-loop command: it brings the tab to the foreground, shows the help overlay, and waits until the user acts or the call times out (default 5m). On unattended / background-server deployments there is no human around, and a blocking help prompt must never appear — but today there is no way to turn the command off.

What changes

New environment variable BSK_REQUEST_HELP:

• BSK_REQUEST_HELP=off (case-insensitive, surrounding whitespace ignored) disables request-help.
• Unset or any other value keeps the current default behaviour (enabled).

When disabled, bsk request-help returns immediately — no daemon startup, no overlay, no waiting — with a synthetic structured result and exit code 0:

{ "outcome": "disabled", "note": "request-help disabled by BSK_REQUEST_HELP=off (unattended mode)", "tab_id": 0 }

The agent therefore gets a normal, machine-readable result instead of an error and can react deliberately (the skill docs now tell agents not to retry request-help after a disabled outcome, and to continue autonomously or stop gracefully).

Implementation

• bsk-protocol: new HelpOutcome::Disabled variant ("disabled") + regenerated tool_request_help_result.json schema.
• CLI (human_loop.rs): dispatch() short-circuits before ensure_daemon() when disabled; the off value parsing lives in a pure, unit-tested helper.
• Daemon (ipc.rs): handle_tool_dispatch refuses to forward tool.request_help to the extension when disabled and answers with the same synthetic result, so non-CLI IPC callers are covered too.
• Extension transport/types.ts: HelpOutcome union gains "disabled" (no behaviour change — the daemon never forwards the call when disabled).
• Docs: both skill/SKILL.md copies document the variable's purpose (unattended mode), its exact behaviour, and the agent guidance above.

Tests

• cargo fmt --all, cargo clippy -p bsk-protocol -p bsk --all-targets -- -D warnings: clean.
• cargo test -p bsk-protocol: 104 passed.
• cargo test -p bsk: all green except one pre-existing, environment-dependent failure in skill_install::sync::tests::sync_continues_on_partial_error (also fails on a clean tree in this environment; unrelated to this change).
• New unit tests cover the off value detection and disabled outcome serialization.