feat(harness): add support for plugins - #29

Two threads on the plugin branch.

Host cwd is now passed, never discovered. `process.cwd()` meant two
different things depending on who read it -- the OS process's directory, or
a session's sandbox mount -- and every consumer guessed. `Harness.layer`
now captures it once as `hostCwd` and threads it down; `Settings.Options.cwd`,
`PluginLoader.Options.hostCwd` and `SandboxDriverLoader.Options.hostCwd` are
required, so omitting one is a type error rather than a silent host read.

`Settings.layer` had never been passed a directory at all and resolved
against `process.cwd()` on its own. That worked only because the values
happened to agree; they stop agreeing the moment a layer is built from
elsewhere.

`sandbox/io.ts` and `sandbox/control.ts` keep theirs. `SandboxIO.host()`
means "the sandbox that is the host process", so the process directory is
its definition, not a fallback -- and the controller's local root must agree
with it. Parameterizing one half of that pair would let them diverge.

Plugin hardening: `setup` no longer double-wraps a typed SetupError;
`idPattern` requires exactly three segments; `validate` returns the author's
object rather than the Struct-decoded copy, which was stripping undeclared
properties and repointing `this` inside the `setup() {}` shorthand;
`guidelines.ts` folds into the default prompt plugin; `ToolRegistration`
moves to `tool/schema.ts` so the executor shares the contract without
importing plugin assembly; `errorMessage` is shared by the executor and the
loop's outer catch so one defect reads the same either way.

…ckage smoke

Plugin fixtures move to typed tool/, prompt/, event/, and host/ modules; malformed ones stay invalid so runtime
validation is still covered. Adds shared remote Bash coverage through Harness plus tool output retention tests, and
moves errorMessage to src/util so the loop and executor share it. The package smoke now declines native build scripts
explicitly and pins @effect/platform-node-shared, whose caret range drifts across prerelease tags.

Co-Authored-By: Claude Opus 5 <[email protected]>

…rojects

Remove the config subdirectory from the harness home, so user settings
live at ~/.codework/settings.json directly. The project layer now
resolves codework.json first and falls back to .codework/settings.json,
so a project can use a single file or a directory — never both.