Add next-turn parameters and tool-call approval to the agent loop

[HiLleywyn]

Summary

Extends the agent loop with two within-turn controls from the OpenRouter Agent SDK, implemented so the sidecar stays stateless per turn (the test_sidecar_guards.py guard still holds) and conversation state stays in the bot.

• Next-turn parameters. A tool may return a next_turn block in its result to steer the following model turn: a different model, a new temperature, a tighter max_output_tokens, or extra instructions. The sidecar wires every bridged tool to the SDK's nextTurnParams and forwards the directive on the tool_result frame; the in-process loop applies the same directive directly. Both paths honour the identical four parameters.
• Tool-call approval. A tool call can be gated on a human yes/no. A gated call posts an Approve / Reject prompt in the channel and waits for the person who asked; a rejected call never runs and the model is handed a declined result so the turn continues. Gating is opt-in: tool names in AGENT_APPROVAL_TOOLS, risk tiers in AGENT_APPROVAL_RISKS, or requires_approval on a ToolSpec. With both config lists empty (the default) nothing is gated and behaviour is unchanged. Approval is resolved entirely on the bot side, and fails closed when no approver is reachable.
• State. Per the chosen architecture, conversation state stays in the bot's existing stores; approval decisions are tracked within the turn and logged. No persistent SDK state surface is added to the sidecar.

The shared logic lives in the new ai/agent_control.py, used by both the in-process loop and the sidecar bridge. The wire protocol is bumped to v2 for the optional next_turn field; a version mismatch falls a turn back to the in-process loop as before, and that loop fully supports both features.

Changes

• ai/agent_control.py (new) -- next_turn directive validation/application and the approval gate.
• agent-sidecar/src/server.ts -- wires nextTurnParams, reads next_turn off tool_result, protocol v2.
• ai/agent_sidecar.py, ai/tools.py -- approval gate plus next_turn handling on both agent paths; ToolSpec.requires_approval, ToolContext.approver.
• cogs/chat.py, cogs/chat_views.py -- the Discord Approve / Reject prompt and renderer status line.
• config.py, .env.example, README.md -- new AGENT_APPROVAL_* settings and docs.

Test plan

• python -m pytest tests/ -- 157 passing, including the new tests/test_agent_control.py and the unchanged sidecar statelessness guard.
• npm run build in agent-sidecar/ -- type-checks and builds clean.
• python -m compileall config.py main.py framework ai cogs.
• Live: a gated tool posts an Approve / Reject prompt; reject keeps the tool unrun; a next_turn directive retunes the following turn.

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