feat(host): add host command namespace for env-injected execution

[roodboi]

Summary

• add a top-level hack host namespace for host-side env-injected execution
• prefer --scope for host execution so env selection is separate from container execution
• clarify docs around when to use hack host, hack run, and compose exec

Verification

• bun test tests/env-exec-command.test.ts tests/cli-command.test.ts
• bun run typecheck
• bun x ultracite check src/commands/env.ts src/cli/spec.ts tests/env-exec-command.test.ts tests/cli-command.test.ts README.md docs/env.md docs/cli.md docs/integrations.md docs/guides/codex-managed-environments.md src/mcp/agent-docs.ts AGENTS.md

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Latest commit: c9bb856

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