feat: live stack-state injection via agent harness hook

[aarontrowbridge]

What

The agent's system prompt currently gets solver mode and routing guidance only at file-generation time (boot / solver-switch / vault-respawn). This adds a second opencode plugin (amicode_context.ts) implementing experimental.chat.system.transform that reads the live filesystem state on every prompt build and injects a ## Stack state (live) block.

Changes

• opencode-plugin/stack_state.ts — standalone state readers + markdown builders (solver mode, routing, active problem, live runs), node-builtins only for Bun runtime
• opencode-plugin/amicode_context.ts — new plugin, single export, one hook
• src/opencode_config.ts — removed solver/routing from AGENTS.md file splice (replaced with recovery pointer); buildOpencodeConfigContent accepts extraPluginPaths param
• src/extension.ts — boot + solver-switch both register the context plugin; solver-switch now passes projectDir (fixes mkdtemp asymmetry bug)
• ~/.amico/server/opencode-config-content.json — context plugin registered for launchd server
• packages/extension/AGENTS.md — recovery pointer appended

Verification

Typecheck clean, all 909 tests pass (85 test files).