feat: make registered commands shell-native

[M09Ic]

Summary

• make registered in-memory commands automatically available inside real shell expressions
• keep simple commands on the existing in-process tmux CreateFunc path; allocate the shell adapter lazily only for real shell execution
• let the platform shell own &&, ||, pipes, redirects, command substitution, and nested-shell semantics
• keep PTY/tmux as the sole owner of sessions, output capture, timeout, kill, and background monitoring
• preserve the original Go context for commands invoked from shell expressions
• dispatch private PATH shims inside pkg/commands, with no host main, CLI flag, or YAML configuration
• cancel active calls and remove per-process runtime files on close; reclaim exited-process leftovers on startup

Runtime model

simple registered command -> existing tmux CreateFunc -> CommandRegistry
shell expression           -> existing PTY/tmux -> real shell -> private PATH shim -> CommandRegistry

The local adapter only carries one command invocation's stdin/stdout/stderr, context identity, and exit status. It does not parse shell syntax or own a second session lifecycle.

Safety

• no global PATH changes
• no registry, shell profile, service, or firewall changes
• adapter variables and PATH entries exist only in the child shell environment
• Unix runtime/socket permissions are owner-only
• Windows named pipes are restricted to the current user
• no runtime directory is created until a real shell path is used

Validation

Windows:

• go test ./pkg/commands ./pkg/runner ./core/config ./cmd/aiscan -count=1
• go test -race ./pkg/commands -run 'TestShellCommand|TestCoreFactory' -count=1

WSL2 Ubuntu:

• GOTOOLCHAIN=go1.26.1 GOPROXY=https://goproxy.cn,direct go test ./pkg/commands ./pkg/runner ./core/config ./cmd/aiscan
• GOTOOLCHAIN=go1.26.1 GOPROXY=https://goproxy.cn,direct go test -race ./pkg/commands -run 'TestShellCommand|TestCoreFactory' -count=1