Add non-secret shell floor for terminals agents and non-interactive zsh

## Parent

Parent: #1

## What to build

Add the non-secret shell floor that makes terminals, agents, and non-interactive zsh commands see the same portable environment without loading secrets or interactive shell behavior.

This slice should add a shared POSIX-compatible environment core and a tiny universal zsh startup path. The result should make aqua binaries and mise shims discoverable for `zsh -c` commands while staying silent, idempotent, and non-secret.

## Acceptance criteria

- [ ] A managed shared environment core sets XDG defaults, the aqua global config location, and the portable PATH floor.
- [ ] PATH mutation is idempotent and avoids duplicate entries across repeated shell startup.
- [ ] Aqua's bin path is available before mise shims/activation assumptions.
- [ ] Universal zsh startup sources only the shared environment core when readable.
- [ ] Universal zsh startup does not run Homebrew, full mise activation, 1Password, prompt setup, completions, aliases, or any interactive/prompting command.
- [ ] The current ambient 1Password secret-loading pattern is removed from universal shell startup.
- [ ] macOS Homebrew PATH behavior is gated to the appropriate host/OS/profile behavior.
- [ ] A fresh non-interactive zsh invocation can discover the expected portable tool paths without output, prompts, or secret loading.
- [ ] No managed shell file contains plaintext secrets, service-account tokens, private keys, or resolved 1Password values.

## Blocked by

- #3