docs(packaging): note start-cli attach subcontainer selection

Document that `package attach` selects a subcontainer by name with `-n`
(or image with `-i`); `-s` matches the internal Guid, not the name. Note
the interactive-picker panic in a non-TTY shell is a missing selector,
not a TTY requirement.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: MattDHill

packaging/package-template/AGENTS.md

@@ -7,3 +7,7 @@ which provides the packaging guide and agent context one level up. If you're rea
 bare clone with no workspace, the full guide is at <https://docs.start9.com/packaging>.
 
 Work this package's `TODO.md` from top to bottom. Keep `README.md` (architecture, for developers and LLMs) and `instructions.md` (end-user docs) in sync with your changes.
+
+## Inspecting a running install
+
+To run a command inside a service's container (read its generated config, grep app logs), use `start-cli package attach <id> -n <subcontainer-name> -- <cmd>`. Select the subcontainer by **name** with `-n` (the name passed to `SubContainer.of` in `main.ts`, e.g. `-n web`) or by image with `-i`. Note: `-s/--subcontainer` matches the internal **Guid**, not the name, so passing a name to `-s` fails with "no matching subcontainers". A service with more than one subcontainer requires a selector; with none given, `attach` falls back to an interactive picker that panics in a non-TTY shell — that's the missing selector, not a TTY requirement.