Wire SproutGit's MCP server into every integrated agent session

[liam-russell]

Problem

chat.ts opens every ACP session with mcpServers: []. Integrated agents get no SproutGit tools at all — an agent that wants to create a worktree, check PR state, or fan out subtasks has to shell out to raw git/gh and hope for the best. All the agent-facing capability we're building (#137 fan-out, #138 PR lifecycle, #135 base-aware worktree creation) would otherwise only be reachable by humans.

Proposal

• Auto-inject SproutGit's MCP server (relates Expand and document the MCP server tools #99) into every ACP session, scoped to the calling workspace + worktree.
• Capability gating by role: an orchestration worktree's agent (Orchestration worktrees: give the planner agent a checkout and a branch to integrate into #134) gets planning tools (fan-out, worktree creation, stack status); a worker's agent gets its-own-PR tools (PR lifecycle depth: draft/ready, per-check results, merge — for humans and agents #138) and status reporting. Everything respects the per-workspace agent-permission setting (auto / ask / never).
• For terminal-mode agents, expose the same server via env (SPROUTGIT_MCP_URL or a socket path) so CLIs that support MCP config can attach.

This is the piece that makes the orchestration epic agent-agnostic: any ACP-speaking agent — including ad-hoc ones from the roster issue — gets the same tools as the built-in presets, with zero per-agent integration work.