Feature-gated Wasm host tools for the CLI

[cachebag]

This adds --tool name=./handler.wasm support behind a new wasm-host-fns Cargo feature. The CLI now can register WASIp1 modules as custom host tools and route guest __dispatch calls to them through the existing ToolRegistry path.

Each Wasm tool is compiled and linked before the VM boots, then invoked as a fresh WASIp1 command per tool call. The handler gets the normal dispatch envelope on stdin, {"name":"...","args":...}, and writes either a raw JSON result or a {"result": ...} / {"error": "..."} envelope to stdout. WASI filesystem and env access are capability-based and off by default, with explicit --tool-wasi-dir, --tool-wasi-dir-ro, --tool-wasi-env, and --tool-wasi-env-inherit flags. Calls also have fuel and stdout/stderr capture limits.

Closes #84

[Copilot]

Pull request overview

Note

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Adds support for registering and invoking host-side custom tools implemented as WASI Preview1 WebAssembly modules (behind an optional Cargo feature), including CLI flags and documentation updates.

Changes:

• Introduces a wasm-host-fns feature with optional wasmtime/wasmtime-wasi dependencies and a new Wasm tool runner (WasmTool).
• Extends the host CLI to register --tool NAME=WASM tools and configure their WASI capabilities/limits (--tool-wasi-*).
• Updates docs and README to describe the new tool mechanism and its security/limits model.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
host/src/wasm_host_fns.rs	New Wasm tool loader/invoker with WASI Preview1 wiring, option parsing, stdout parsing, and tests.
host/src/main.rs	Adds feature-gated CLI flags for Wasm tools and registers tools into SandboxBuilder.
host/Cargo.toml	Adds wasm-host-fns feature and optional wasmtime / wasmtime-wasi deps.
docs/host_functions.md	Documents --tool and --tool-wasi-* behavior, protocol, and limits.
README.md	Updates dispatch surface description and adds usage docs for Wasm host tools.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated 5 comments.

[danbugs]

This is AWESOME!! Thank you so much for the contribution! Just ggot some comments here and there for you to consider.

[danbugs]

Mostly LGTM. I think this should be good to merge after addressing the echo tool comment. Thanks!

[danbugs]

Thanks for rebasing. Mostly LGTM. I think we just need to remove leftover --enable-tools references from the host functions doc and main README as well as modify the existing python-host-tools example to use a wasm host function. You can commit the echo Wasm host fxn to the repo under smt like: examples/echo-wasm-host-fxn or smt like that.