v0.2.6 - Daemon auto-update

TL;DR

• Daemon auto-updates itself - downloads, verifies (minisign + SHA256), swaps binaries, and restarts automatically every ~6 hours.
• macOS + Linux - works with launchd (macOS) and systemd user services (Linux).
• Opt-out available - set AGENTJAIL_AUTO_UPDATE=false to disable.
• Shared update infrastructure - download/verify/swap code extracted to internal/selfupdate/ for reuse.

Added

• Daemon auto-update - the daemon periodically checks for new versions and applies updates automatically (download → verify signature → verify hash → swap binaries → exit → service manager restarts)
• Linux systemd support - auto-update works with systemctl --user restart on Linux, in addition to launchd on macOS
• internal/selfupdate/ package - shared download, verification, and binary swap infrastructure used by both CLI and daemon
• Signing key via ldflags - SigningPubKey moved to shared package, injected at build time
• ADR 0026 - documents the daemon auto-update decision

Changed

• agentjail update (CLI) - refactored to use shared internal/selfupdate/ functions
• install.go - launchctl helpers now delegate to shared package

Security

• Same minisign + SHA256 verification chain as manual agentjail update
• Dev builds (empty signing key) skip auto-update
• Homebrew installations skip auto-update (notification only)