[codex] Pluggable Tool Registry

[aboimpinto]

This adds the third stacked layer on top of Hmbown#2294: pluggable tool registry support for runtime registration, config.toml overrides, and plugin discovery.

What changed:

• restored the existing tools_always_load behavior from the stacked shell branch
• added [tools] support for plugin_dir and per-tool overrides
• registered plugin tools as model-visible tools without deferring user-added overrides/plugins
• added registry/plugin regression tests

Why:

• the owner asked for the feature to be split into stacked PRs rather than one broad monolith
• this keeps registry/plugin changes reviewable on top of the shell and external-tool layers

Validation:

• cargo fmt --all
• cargo check -p codewhale-tui --all-features --locked
• cargo test -p codewhale-tui test_parse_frontmatter_full -- --nocapture
• cargo test -p codewhale-tui test_load_plugin_tools_creates_tools -- --nocapture
• cargo test -p codewhale-tui test_tool_from_override_script -- --nocapture

Paulo Aboim Pinto