codex-tools: extract collaboration tool specs (#16141)

## Why

The recent `codex-tools` migration steps have moved shared tool models
and low-coupling spec helpers out of `codex-core`, but
`core/src/tools/spec.rs` still owned a large block of pure
collaboration-tool spec construction. Those builders do not need session
state or runtime behavior; they only need a small amount of core-owned
configuration injected at the seam.

Moving that cohesive slice into `codex-tools` makes the crate boundary
more honest and removes a substantial amount of passive tool-spec logic
from `codex-core` without trying to move the runtime-coupled multi-agent
handlers at the same time.

## What changed

- added `agent_tool.rs`, `request_user_input_tool.rs`, and
`agent_job_tool.rs` to `codex-tools`, with sibling `*_tests.rs` coverage
and an exports-only `lib.rs`
- moved the pure `ToolSpec` builders for:
- collaboration tools such as `spawn_agent`, `send_input`,
`send_message`, `assign_task`, `resume_agent`, `wait_agent`,
`list_agents`, and `close_agent`
  - `request_user_input`
  - agent-job specs `spawn_agents_on_csv` and `report_agent_job_result`
- rewired `core/src/tools/spec.rs` to call the extracted builders while
still supplying the core-owned inputs, such as spawn-agent role
descriptions and wait timeout bounds
- updated the `core/src/tools/spec.rs` seam tests to build expected
collaboration specs through `codex-tools`
- updated `codex-rs/tools/README.md` so the crate documentation reflects
the broader collaboration-tool boundary

## Test plan

- `CARGO_TARGET_DIR=/tmp/codex-tools-collab-specs cargo test -p
codex-tools`
- `CARGO_TARGET_DIR=/tmp/codex-core-collab-specs cargo test -p
codex-core --lib tools::spec::`
- `just fix -p codex-tools -p codex-core`
- `just argument-comment-lint`

## References

- #15923
- #15928
- #15944
- #15953
- #16031
- #16047
- #16129
- #16132
- #16138

codex-rs/tools/README.md

@@ -23,6 +23,8 @@ schema and Responses API tool primitives that no longer need to live in
 - `ResponsesApiNamespaceTool`
 - code-mode `ToolSpec` adapters
 - local host tool spec builders for shell/exec/request-permissions/view-image
+- collaboration and agent-job `ToolSpec` builders for spawn/send/wait/close,
+  `request_user_input`, and CSV fanout/reporting
 - `parse_tool_input_schema()`
 - `parse_dynamic_tool()`
 - `parse_mcp_tool()`