Add plugin host version checks

[i386]

Summary

Adds a plugin.toml host compatibility gate so installed plugins can declare the minimum mesh-llm version they require.

Changes

• Parse min_mesh_llm_version from plugin manifests, with aliases for older naming experiments.
• Reject incompatible plugin archives during install before replacing an existing install.
• Re-check installed plugins at runtime and mark incompatible plugins inactive instead of launching them.
• Re-check installed plugin CLI commands before dispatch.
• Document the manifest field in the plugin packaging docs.

CLI Experience

Install rejects an incompatible archive before replacing any existing plugin install:

$ mesh-llm plugins install mesh-llm/cool-plugin
🚫 Error: 🚫 Plugin requires mesh-llm >= 0.69.0, current host is 0.68.0

Listing installed plugins shows an already-installed incompatible plugin as inactive, with the same minimum-version reason:

$ mesh-llm plugins list
cool-plugin	kind=installed	state=incompatible	error=🚫 Plugin requires mesh-llm >= 0.69.0, current host is 0.68.0

Running a plugin-provided CLI command fails before launching the plugin process:

$ mesh-llm cool-plugin run
🚫 Error: 🚫 Plugin command 'cool-plugin' is not compatible

Caused by:
    🚫 Plugin requires mesh-llm >= 0.69.0, current host is 0.68.0

Configured [[plugin]] command = "..." entries are intentionally outside this manifest check because they do not install a plugin.toml; those still fail through initialize/protocol compatibility checks.

Validation

• cargo test -p mesh-llm-plugin-manager --lib
• just build
• cargo test -p mesh-llm-host-runtime cli::commands::plugin_cli --lib
• cargo check -p mesh-llm
• rustfmt --check on changed Rust files
• git diff --check