docs: download latest ado-aw release in agent setup instructions

[Copilot]

Summary

Drops the pinned VERSION from the user-facing agent install snippets in favor of GitHub's releases/latest/download/{filename} URL, so freshly initialized agents always pick up the most recent compiler release without coordinated bumps.

• src/data/init-agent.md: rewrite Linux/macOS/Windows Setup blocks to fetch ado-aw-* and checksums.txt from https://github.com/githubnext/ado-aw/releases/latest/download/...; remove the VERSION shell/PowerShell variable. Other {{ compiler_version }}-pinned URLs (prompt files, AGENTS.md) are intentionally untouched — those reference repo content at a tag, not binaries.
• .github/agents/ado-aw.agent.md: same change applied to the checked-in dispatcher agent (previously hardcoded VERSION="0.16.0").
• Out of scope: pipeline templates (src/data/base.yml, src/data/1es-base.yml) keep {{ compiler_version }} pinning for reproducible builds.

Example of the new install line:

curl -fsSL -o /tmp/ado-aw "https://github.com/githubnext/ado-aw/releases/latest/download/ado-aw-linux-x64"
curl -fsSL -o /tmp/checksums.txt "https://github.com/githubnext/ado-aw/releases/latest/download/checksums.txt"
cd /tmp && grep "ado-aw-linux-x64" checksums.txt | sha256sum -c - && chmod +x /tmp/ado-aw

Test plan

• cargo build, cargo test (incl. tests/init_tests.rs which asserts {{ compiler_version }} is substituted — still passes since the template retains that marker for prompt URLs), cargo clippy --all-targets --all-features — all green.