Add ADR-005: Ergonomic tool definition via annotation processor (#1684)

Introduces an Architecture Decision Record documenting the plan to add a
langchain4j-style annotation-on-method API for defining Copilot tools in Java.

## Problem

The current `ToolDefinition.create()` API requires manually constructing JSON
Schema parameter objects—verbose and error-prone compared to the ergonomic
patterns available in other Java AI frameworks (langchain4j `@Tool`/`@P`,
Spring AI `@Tool`/`@ToolParam`).

## Decision

Adopt Option 3 (annotation-on-method) with a JSR 269 annotation processor:

- `@CopilotTool` on methods defines tools; method name → tool name,
  Javadoc/annotation attribute → description.
- `@Param` on parameters provides descriptions and constraints; the processor
  reads source-level parameter names at compile time, eliminating the need for
  the `-parameters` javac flag.
- A compile-time annotation processor generates `ToolDefinition` instances and
  JSON Schema from annotated methods—no runtime reflection required.
- The new API surface is gated behind `@CopilotExperimental` (ADR-004) until
  stabilized.
- The existing `ToolDefinition.create()` / `CopilotTools.define()` APIs remain
  unchanged and fully supported.

## Scope

This ADR captures the design decision only. Implementation will follow in a
separate PR tracked by #1682.

Related: #1682

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: edburns