Fix constrained deny-list tool declarations

[trotsky1997]

Fixes #22.

Summary

• keep constrained allow-list declarations fail-closed so they do not expose the whole tool
• make constrained deny-list declarations fail closed by blocking the matching head tool name
• add a slash-runner regression test for disallowed-tools: Bash(git push:*)

Testing

• cargo fmt --check
• git diff --check -- src/session.rs tests/session_runner_slash.rs
• attempted cargo test --test session_runner_slash constrained, but local dependency fetch for https://github.com/Dicklesworthstone/opentui_rust timed out/disconnected with fetch-pack: unexpected disconnect while reading sideband packet

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