✨ Add standalone binary releases with install scripts

[renardeinside]

Summary

• Embed apx-agent into the apx binary via include_bytes! (same pattern as bun), producing a single self-contained binary per platform
• Add build-standalone CI job that builds standalone binaries for all 5 platforms (Linux x86_64/aarch64, macOS x86_64/aarch64, Windows x64) and attaches them to GitHub Releases alongside wheels
• Create curl | sh / irm | iex install scripts (docs/public/install.sh and docs/public/install.ps1) for direct binary distribution without needing uv
• Deduplicate build.rs — extract shared copy_platform_binary() for bun and agent embedding
• Simplify agent.rs — ensure_installed() now delegates to embedded extraction instead of manual copy logic

Changes

File	What
crates/core/build.rs	Add copy_agent_binary() via shared copy_platform_binary()
crates/core/src/resources.rs	Add AGENT_BINARY const + ensure_agent_extracted()
crates/core/src/interop.rs	Use embedded extraction as priority 2 in agent resolution
crates/core/src/agent.rs	Simplify to delegate to resolve_apx_agent_binary_path()
.github/workflows/release.yml	Add build-standalone job, update create-release
docs/public/install.sh	POSIX sh installer for Linux/macOS
docs/public/install.ps1	PowerShell installer for Windows

Test plan

• just check passes (fmt, clippy, type check)
• cargo build -p apx-bin --release produces a self-contained binary
• Running the binary extracts agent to ~/.apx/apx-agent automatically
• Push a test tag to verify build-standalone CI job produces 5 binaries
• Verify install scripts work: curl -fsSL .../install.sh | sh -s -- --version <tag>
• Verify existing wheel build pipeline is unaffected

🤖 Generated with Claude Code