feat(deepagents): simplify prompting (#703)

## Summary

The SDK currently ships authored base prose plus middleware guidance
that largely duplicates the tool schemas. This change adopts a more lean
default: no authored base prompt, no duplicate built-in usage prose, and
concise tool descriptions.

Skills, memory, custom system prompts, custom tool descriptions, and
required filesystem routing context remain intact. The legacy
base-prompt surface is removed rather than preserved as a default path.

## Changes

### Default prompt

- Remove the authored default base prompt and its obsolete public
surface.
- Suppress LangChain's default todo middleware prompt while leaving
deepagents-owned middleware prompt-free by default.
- Remove unused filesystem, subagent, async-subagent, and execution
prompt constants and exports.

### Durable tool guidance

- Keep large-result recovery guidance in `read_file` and `grep`
descriptions.
- Keep async task status freshness guidance in `check_async_task` and
`list_async_tasks` descriptions.
- Trim filesystem and task descriptions to their purpose and
load-bearing constraints.
- Build grep and execute descriptions from the configured filesystem
tool set so they do not recommend unavailable tools.

### Token impact

Offline (`o200k_base`, default-agent tool schemas):

| Tool | Before | After | Delta |
| --- | ---: | ---: | ---: |
| `task` | 1,664 | 389 | -77% |
| `read_file` | 728 | 494 | -32% |
| `grep` | 688 | 555 | -19% |
| `edit_file` | 273 | 257 | -6% |
| `glob` | 217 | 172 | -21% |
| `write_file` | 177 | 177 | — |
| `delete` | 146 | 146 | — |
| `ls` | 111 | 111 | — |
| **Total** | **4,005** | **2,302** | **-43%** |

For a default-agent `hello` turn, input tokens drop from 5,395 to 1,895
(-65%).

### Evaluation results

The lean prompt matched the baseline on correctness (0.81) and solve
rate (0.635 vs. 0.634). Unified evaluation macro and micro scores
improved from 0.413 to 0.437 and from 0.371 to 0.394, respectively. (see
more detailed results in
https://github.com/langchain-ai/deepagents/pull/5009 and
https://github.com/langchain-ai/deepagents/pull/4859)