feat: help text lists only 28 builtins but 109 are implemented

[chaliy]

Summary

The `BashTool` help text and system prompt only list 28 builtins:

```
echo cat grep sed awk jq curl head tail sort uniq cut tr wc date sleep mkdir rm cp mv touch chmod printf test [ true false exit cd pwd ls find xargs basename dirname env export read
```

But 109 builtins are actually implemented. This means agents don't know they can use `tar`, `gzip`, `stat`, `base64`, `seq`, `tee`, `diff`, `comm`, `column`, `git`, `python`, `jq`, `rev`, `tac`, `expr`, `nl`, `od`, `xxd`, `strings`, `file`, `realpath`, `mktemp`, `timeout`, `getopts`, `declare`, `pushd`/`popd`, etc.

Impact

Agents will use workarounds (like awk-based hacks) instead of purpose-built builtins they don't know about. This directly reduces agent capability.

Location

`crates/bashkit/src/tool.rs:46`:
```rust
const BUILTINS: &str = "echo cat grep sed awk jq curl head tail sort uniq cut tr wc date sleep mkdir rm cp mv touch chmod printf test [ true false exit cd pwd ls find xargs basename dirname env export read";
```

And `BASE_HELP` at line 49 repeats the same short list.

Fix

Update `BUILTINS` const and help text to include all 109 builtins, organized by category as in `lib.rs` line 20-34.

Acceptance criteria

• `BUILTINS` const lists all implemented builtins
• `help()` output organized by category
• `system_prompt()` includes full builtin list
• Custom builtins (from `BashToolBuilder`) still appear in help