feat(subagents): complete work_dir override for Shell cwd and AGENTS.md context

Background

PR #1933 added work_dir override for subagents, fixing #1931. Two related concerns were identified during code review but intentionally deferred to keep the initial PR scope manageable.

Issue 1: Shell tool uses session.work_dir instead of overridden work_dir

The Shell tool resolves its cwd from session.work_dir, which is never overridden when work_dir is set on a subagent. This means git and build commands run by subagents with a custom work_dir may execute in the parent's working directory instead of the intended one.

• src/kimi_cli/tools/shell/__init__.py uses self._runtime.session.work_dir for background runs
• Foreground runs inherit the process cwd set from session.work_dir in src/kimi_cli/app.py
• Only builtin_args.KIMI_WORK_DIR is overridden, not the session's work_dir

Why deferred: Changing Shell cwd resolution affects all agents and is out of scope for the initial work_dir feature. The parameter currently targets file tools (Read, Write, Glob) and system prompt context.

Issue 2: KIMI_AGENTS_MD not reloaded for overridden work_dir

When work_dir_override is set, KIMI_AGENTS_MD is not refreshed from the new directory. The subagent retains the parent's AGENTS.md content in its system prompt.

• Reloading AGENTS.md requires _find_project_root detection and filesystem scanning
• The subagent can discover AGENTS.md through its own tool usage
• Only KIMI_WORK_DIR_LS (directory listing) is cheap to refresh and provides immediate value

Why deferred: Cost/benefit tradeoff — directory listing refresh is cheap and high-value; AGENTS.md reload adds complexity for marginal gain since the subagent discovers it naturally.

Scope

• Investigate whether session.work_dir should also be overridden (or Shell should read from builtin_args.KIMI_WORK_DIR)
• Add optional AGENTS.md reload in build_builtin_instance when work_dir_override is set
• Add tests for Shell cwd behavior with overridden work_dir

Related: #1931, #1933

Start with src/kimi_cli/tools/shell/init.py and src/kimi_cli/app.py to trace foreground and background cwd handling, then locate build_builtin_instance and the existing work_dir tests. Verify behavior for a subagent override, decide how AGENTS.md should be refreshed, and add tests covering the intended Shell cwd and context.