[codex] Move tool specs into core handlers

[pakrym-oai]

Why

This is the first mechanical slice of moving tool spec ownership toward the handlers. codex-tools should keep shared primitives and conversion helpers, while builtin tool specs and registration planning live in codex-core with the handlers that own those tools.

Keeping this PR to relocation and import updates isolates the copy/move review from the later logic change that wires specs through registered handlers.

What changed

• Moved builtin tool spec constructors from codex-rs/tools/src into codex-rs/core/src/tools/handlers/*_spec.rs or nearby core tool modules.
• Moved the registry planning code into codex-rs/core/src/tools/spec_plan.rs and its associated types/tests into core.
• Kept shared primitives in codex-tools, including ToolSpec, schema/types, discovery/config primitives, dynamic/MCP conversion helpers, and code-mode collection helpers.
• Updated handlers that referenced moved argument types or tool-name constants to use the core spec modules.
• Moved spec tests next to the moved spec modules.

Verification

• cargo check -p codex-tools
• cargo check -p codex-core
• cargo test -p codex-tools
• cargo test -p codex-core _spec::tests
• cargo test -p codex-core tools::spec_plan::tests
• just fix -p codex-tools
• just fix -p codex-core

Note: I also tried the broader cargo test -p codex-core tools::; it reached the moved spec-plan/spec tests successfully, then aborted with a stack overflow in tools::handlers::multi_agents::tests::tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtrees_closed, which is outside this spec relocation.