Per-plugin toggle for exposing CLI commands to agents (plugin-commands skill curation)

[swairshah]

Problem

Every plugin that registers a CLI shows up in the server-generated plugin-commands skill, so agents see all plugin commands. As the plugin count grows this becomes noise: not every plugin's commands are useful to agents, and today the only way to hide them is disabling the plugin entirely.

Change

Adds a per-plugin agent CLI exposure toggle (default: exposed, preserving current behavior). A hidden plugin keeps a fully working CLI — its commands are just omitted from the generated plugin-commands skill, so agents never discover them.

• DB: plugins.agent_cli_exposed boolean column (migration 0110_plugin_agent_cli_exposed), preserved across reinstalls/updates and state-snapshot rollbacks.
• Server: cliContributions() filters hidden plugins before writing the skill; PluginService.setAgentCliExposed() re-syncs the skill and broadcasts plugins-changed. New route POST /api/v1/plugins/:id/agent-cli with { exposed: boolean }.
• CLI: bb plugin agent-cli <id> [on|off] shows or sets the toggle; bb plugin list marks hidden commands with "(hidden from agents)".
• SDK: plugins.setAgentCliExposed(); InstalledPlugin.agentCliExposed (schema default true, so older servers parse fine).
• App: an "Agent access" section with a switch on the plugin detail page, shown when the plugin registers a CLI command.
• Docs: bb-cli command index + plugins reference, plugin guide template, and the plugin-authoring skill's CLI chapter.

Tests

• New server test: toggling off removes the plugin's section from the skill (and removes the skill file when no contributions remain), the command stays runnable, toggling back regenerates it, plus 400/404 request validation.
• Migration rewind tests updated with a drop agent_cli_exposed helper, mirroring the existing pattern.
• Full turbo run typecheck (80 tasks) passes; db/sdk/cli/server plugin suites and app plugin component tests pass. The only failing test locally is install-machine-script.test.ts, which also fails on a clean main checkout (environment-dependent).