codex-tools: extract discovery tool specs

[bolinfest]

Why

core/src/tools/spec.rs still owned the pure tool_search and tool_suggest spec builders even though that logic no longer needed codex-core runtime state. This change continues the codex-tools migration by moving the reusable discovery and suggestion spec construction out of codex-core so spec.rs is left with the core-owned policy decisions about when these tools are exposed and what metadata is available.

What changed

• Added codex-rs/tools/src/tool_discovery.rs with the shared tool_search and tool_suggest spec builders, plus focused unit tests in tool_discovery_tests.rs.
• Moved the shared DiscoverableToolAction and DiscoverableToolType declarations into codex-tools so the tool_suggest handler and the extracted spec builders use the same wire-model enums.
• Updated core/src/tools/spec.rs to translate ToolInfo and DiscoverableTool values into neutral codex-tools inputs and delegate the actual spec building there.
• Removed the old template-based description rendering helpers from core/src/tools/spec.rs and deleted the now-dead helper methods in core/src/tools/discoverable.rs.
• Updated codex-rs/tools/README.md to document that discovery and suggestion models/spec builders now live in codex-tools.

Test plan

• cargo test -p codex-tools
• CARGO_TARGET_DIR=/tmp/codex-core-discovery-specs cargo test -p codex-core --lib tools::spec::
• CARGO_TARGET_DIR=/tmp/codex-core-discovery-specs cargo test -p codex-core --lib tools::handlers::tool_suggest::
• just argument-comment-lint

References

• codex-tools: extract utility tool specs #16154
• 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
• codex-tools: extract local host tool specs #16138
• codex-tools: extract collaboration tool specs #16141