fix: ship no agents so VS Code lists none twice

[hackernotfound]

Ships the Copilot build with no plugin agents. VS Code's agents window merges the agents it parses from a plugin with the agents its agent host reads from a synced copy of that plugin, and removes duplicates by file location. The two copies live at different paths, so every plugin agent shows up twice in the agent picker. VS Code's Copilot runtime also can't dispatch plugin agents. Copilot's built-in general-purpose agent, told to load poteto-mode first, already does what pstack:poteto-agent does.

Changes

• tools/build.mjs drops agents/ and effort-agents/ and the manifest's agents list. The startup instructions say no agents ship.
• copilot-tools.md maps every pstack agent name to general-purpose with a fixed opening, reads comment-sicko from skills/poteto-mode/references/agents/, and passes any effort level as reasoning_effort.
• tools/check.mjs fails if the plugin ships any agent again.
• The live checks require that no pstack agents are offered, that a subagent loads a pstack skill, and that no subagent dispatch is rejected.
• README, plugin README, and docs/maintaining.md describe the change. Version 0.9.57-copilot.3.

Verified:

• node tools/check.mjs: 19/19. The new gate fails when an agent is added back through overlay/.
• tools/eval.sh (Copilot CLI 1.0.91): 10/10.
• node tools/eval-vscode.mjs (VS Code 1.140 runtime): 10/10 on two runs. The subagent is general-purpose, opens with the poteto-mode instruction, and loads poteto-mode.

Notes for reviewer

• The duplicate listing comes from getCustomAgents in VS Code's baseAgentHostSessionsProvider.ts, which keys the merge by URI. It affects any plugin that ships agents.