Stable plugin API for kaish tools.

This leaf crate defines the contract that every kaish tool — builtin or third-party — implements, without pulling in kaish-kernel's parser, interpreter, or async runtime internals. It exists so out-of-tree tool bundles (kaish-tools-host, kaish-tools-git, a hypothetical kaish-tools-perl) can be written against a small, audited surface.

The surface

• [Tool] — the trait every command implements. Its execute takes a &mut dyn ToolCtx, so the concrete kernel context never leaks into the public API.
• [ToolCtx] — the trimmed execution context. It exposes only what a well-behaved, portable tool needs (backend I/O, cwd, variable access, output format). Trusted in-tree builtins that need deeper kernel state (job control, pipes, the dispatcher) recover the concrete context via [ToolCtx::as_any_mut] — a documented escape hatch, not part of the portable contract.
• [KernelBackend] — the I/O + tool-dispatch backend a tool reaches through ctx.backend().
• [GlobalFlags], [schema_from_clap], [validate_against_schema] — the clap-reflection and validation machinery shared by all builtins.

The pure-data types tools traffic in (Value, ToolArgs, ToolSchema, ExecResult, OutputData, …) live one layer down in kaish-types.