Add Linux systemd daemon support

[CalebisGross]

Summary

• Extracts hardcoded macOS launchctl calls into a ServiceManager interface with build-tagged implementations per platform
• Adds Linux systemd user service support (systemctl --user) for install, uninstall, start, stop, and status
• Adds unsupported-platform stub so the code compiles cleanly on Windows
• Preserves PID-file fallback for systems without a service manager installed

Files Changed

File	What
internal/daemon/service.go	New ServiceManager interface (7 methods)
internal/daemon/service_darwin.go	macOS launchd — extracted existing code, identical logic
internal/daemon/service_linux.go	Linux systemd — new systemctl --user implementation
internal/daemon/service_other.go	Stub for unsupported platforms
internal/daemon/daemon.go	Removed 48 lines of hardcoded launchctl functions
cmd/mnemonic/main.go	Commands use ServiceManager interface; plist template moved to darwin file
.claude/rules/code-quality.md	New rule: scope discipline, change safety
.claude/rules/platform-safety.md	New rule: never break existing platforms

Test plan

• go build succeeds on Linux (compiles service_linux.go)
• go vet passes
• gofmt clean on all changed files
• All existing tests pass (go test ./...)
• Binary runs: mnemonic version, mnemonic help produce correct output
• Help text updated: "Install as system service" (not "macOS LaunchAgent")
• Verified macOS launchd code is line-for-line identical to the extracted original
• Manual test on macOS: mnemonic install / mnemonic start / mnemonic stop / mnemonic uninstall
• Manual test on Linux: mnemonic install creates systemd unit file

Closes #1, #2, #15

🤖 Generated with Claude Code