CoreCoder v0.6.0: MCP, hooks, plan mode

Four increments since v0.5.0, all in the distilled spirit of the project:

• Engine trim (7f323e7): the core loop got leaner without losing behavior.
• Pre/PostToolUse hooks (d503117): user shell commands around tool calls, configured in ~/.corecoder/hooks.json.
• MCP stdio client (2dfd156): external tools join the agent from ~/.corecoder/mcp.json. Newline-delimited JSON-RPC over subprocess pipes, handshake and crash isolation included. MCP tools flow through the v0.5.0 consent gate and the new hooks like the built-ins.
• Plan mode (caadf45): /plan makes the agent investigate read-only and present a numbered plan before anything mutates. "approve" (or /plan again) hands control back. It outranks even --yes.

146 tests, all green.

中文

自 v0.5.0 以来的四个增量，全部延续项目的「蒸馏」气质：

• 引擎精修（7f323e7）：主循环更瘦，行为不变。
• Pre/PostToolUse 钩子（d503117）：工具调用前后跑用户自己的 shell 命令，配置在 ~/.corecoder/hooks.json。
• MCP stdio 客户端（2dfd156）：从 ~/.corecoder/mcp.json 接入外部工具生态。子进程管道上的逐行 JSON-RPC，带握手与崩溃隔离；MCP 工具和内建工具一样过 v0.5.0 的同意门和新的钩子。
• 计划模式（caadf45）：/plan 让 agent 先只读调查、给出编号计划再动手，approve（或再按 /plan）交还控制权。优先级高于 --yes。

146 个测试全绿。