codex-tools: extract local host tool specs

[bolinfest]

Why

core/src/tools/spec.rs still bundled a set of pure local-host tool builders with the orchestration that actually decides when those tools are exposed and which handlers back them. That made codex-core responsible for JSON/tool-shape construction that does not depend on session state, and it kept the codex-tools migration from taking a meaningfully larger bite out of spec.rs.

This PR moves that reusable spec-building layer into codex-tools while leaving feature gating, handler registration, and runtime-coupled descriptions in codex-core.

What changed

• added codex-rs/tools/src/local_tool.rs for the pure builders for exec_command, write_stdin, shell, shell_command, and request_permissions
• added codex-rs/tools/src/view_image.rs for the view_image tool spec and output schema so the extracted modules stay right-sized
• rewired codex-rs/core/src/tools/spec.rs to call those extracted builders instead of constructing these specs inline
• kept the request_permissions description source in codex-core, with codex-tools taking the description as input so the crate boundary does not grow a dependency on handler/runtime code
• moved the direct constructor coverage for this slice from codex-rs/core/src/tools/spec_tests.rs into codex-rs/tools/src/local_tool_tests.rs and codex-rs/tools/src/view_image_tests.rs
• updated codex-rs/tools/README.md to reflect that codex-tools now owns this local-host spec layer

Test plan

• CARGO_TARGET_DIR=/tmp/codex-tools-local-host cargo test -p codex-tools
• CARGO_TARGET_DIR=/tmp/codex-core-local-tools cargo test -p codex-core --lib tools::spec::
• just argument-comment-lint

References

• codex-tools: extract shared tool schema parsing #15923
• codex-tools: extract MCP schema adapters #15928
• codex-tools: extract dynamic tool adapters #15944
• codex-tools: introduce named tool definitions #15953
• codex-tools: extract responses API tool models #16031
• codex-tools: extract tool spec models #16047
• codex-tools: extract configured tool specs #16129
• codex-tools: extract code mode tool spec adapters #16132