Usability pass: tool registry, TUI polish, saved keys and settings

Tools
- Table-driven tool registry with permission classes and default/minimal
  profiles; new tools: edit_file, list_dir, glob, grep, delete_file,
  move_file, todo_write, web_fetch, ask_user (plus read_file paging).
- Built-in Myers diff; write tools return unified diffs.
- Persistent permission rules (.agent/permissions.json), per-turn undo
  snapshots (.agent/undo), AGENTS.md project notes, auto-compact at 80%
  of the model context, cancellable run_shell with process-group kill.

TUI
- Table-driven slash commands: /provider /key /effort /status /tools
  /permissions /yolo /undo /retry /diff /export /copy /rename /delete
  /init /max-steps /todo, with / and @ completion.
- Permission modal with diff and prefix rules, ask_user modal, todo
  panel, header with model/effort/context, bracketed paste, line editing,
  wide-char cursor, NO_COLOR and ASCII fallback; Esc never quits.

Startup and configuration
- API keys are entered in the TUI (hidden) and saved per provider in
  <config>/credentials (0600); env variables are only a fallback.
- Last provider, per-provider model and reasoning effort are remembered
  in <config>/settings.json; --effort/-e and /effort send
  reasoning_effort (low|medium|high|max) to models that accept it.
- DeepSeek preset uses deepseek-flash / deepseek-v4-pro.
- Session flags --new/--continue/--resume/--list-sessions, --yolo,
  --confirm.

Compatibility and tests
- ncurses detection, macOS/Homebrew support, make install, XDG config
  dir, POSIX cleanups; Gitea CI (gcc/clang/asan/musl/macOS).
- Unit tests for diff, fs tools, registry, undo/perms and config;
  e2e fixtures multi_file_refactor and ask_or_todo; vendored cJSON.

fixtures/makefile_tab/TASK.md

@@ -0,0 +1,11 @@
+# 任务：为 hello 项目写 Makefile
+
+工作区里只有 `hello.c` 和 `tests/expect.txt`，还没有 Makefile。请：
+
+1. 新建 `Makefile`（注意：命令行必须用 TAB 缩进），包含以下目标：
+   - `all`（默认目标）：用 `cc -std=c11 -Wall -Wextra -O2 -o hello hello.c` 编译出 `hello`；
+   - `test`：依赖 `hello`，运行 `./hello > tests/actual.txt`，再用 `diff -u tests/expect.txt tests/actual.txt` 比较；
+   - `clean`：删除 `hello` 和 `tests/actual.txt`；
+   - 把 `all test clean` 声明为 `.PHONY`。
+2. 运行 `make clean && make test`，确认通过。
+3. 用一句话说明结果。

fixtures/makefile_tab/check.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+# Pass iff the agent wrote a working Makefile (recipes need real TABs).
+set -euo pipefail
+cd "$(dirname "$0")"
+test -f Makefile
+grep -q "$(printf '^\t')" Makefile
+make clean >/dev/null
+make test
+make clean >/dev/null
+test ! -e hello

fixtures/makefile_tab/hello.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+
+int main(void) {
+  printf("hello, cst\n");
+  printf("tab\there\n");
+  return 0;
+}

fixtures/makefile_tab/tests/expect.txt

@@ -0,0 +1,2 @@
+hello, cst
+tab	here