[Agent Loop] Tool argument validation (toolArgValidation strict/lenient)

Part of #612.

Implement `toolArgValidation: 'strict' | 'lenient'` — validate model-emitted tool-call arguments against the tool's `parameters` JSON Schema before dispatch. Today the loop throws `501` at entry for anything but the default `'none'`; the `ToolValidationError(400)` class and the option surface are already in place.

## Why deferred

Harper validates internally with Joi and passes JSON Schema through to backends untouched. Strict arg validation needs a JSON Schema validator in-process — adopting Ajv (or equivalent) is a separate dependency/decision, not a loop change.

## Acceptance

- [ ] `'strict'` (the issue's intended default) rejects args that don't satisfy the schema with `ToolValidationError(400)` carrying the validation errors.
- [ ] `'lenient'` coerces/logs but proceeds.
- [ ] `'none'` passes through (current behavior).
- [ ] Tests across each mode.

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