refactor: split monolithic main.go into domain-specific command files

[stuffbucket]

Summary

Split the 2000+ line cmd/coop/main.go into 13 focused files with dependency injection via an App struct.

Changes

File	Lines	Responsibility
main.go	91	Build vars + command dispatch
app.go	113	App struct, NewApp(), Manager(), validators, initLogging()
cmd_container.go	355	init/create/start/stop/lock/unlock/logs/delete/list/status
cmd_shell.go	99	ssh/shell/exec (incus exec path for bladerunner)
cmd_config.go	124	config/env display
cmd_theme.go	230	interactive theme picker and previews
cmd_snapshot.go	151	snapshot CRUD
cmd_mount.go	272	mount add/remove/list with seatbelt auth
cmd_vm.go	117	VM lifecycle commands
cmd_doctor.go	89	health checks
cmd_image.go	223	image build/list/exists/publish/lineage
cmd_state.go	150	state history/show
cmd_help.go	128	help/version with dashboard

Key design decisions

• DI via App struct: All command handlers are methods on *App, which holds *config.Config and BuildInfo. No more appConfig global.
• Lazy Manager(): Sandbox manager is created on first use, not at startup. Commands that don't need it (help, version, config) skip the overhead.
• Zero behavior changes: Pure structural refactoring. All command logic preserved exactly.
• Lint clean: Fixed all 23 errcheck issues across bladerunner, doctor, and cmd files.

Verification

• golangci-lint run: 0 issues
• go vet ./...: clean
• go build ./cmd/coop/: OK
• go test -race ./...: all pass