Fix container hang when Ctrl-C is pressed on apple runtime

[ryanmoran]

What

Fix a hang that occurred when pressing Ctrl-C while running contagent against the apple runtime.

Key changes:

• Wrap the user command in a login shell (/bin/sh -l -c) in Container.Attach so profile files are sourced and PATH includes user-local directories
• Replace direct SIGINT/SIGTERM signal handling in Container.Wait with ctx.Done() so Ctrl-C (which cancels the context in main.go) correctly triggers container shutdown
• Add a 10-second timeout context to ForceRemove in the cleanup handler
• Document the login shell as a temporary workaround, with removal instructions for when apple/container ships a release pinning containerization >= 0.26.5

Why

Two bugs in apple/containerization (PRs #550 and #562) caused container exec to ignore the container's configured PATH and fall back to a hardcoded default that omits user-local directories like ~/.local/bin. When the command couldn't be found, the container hung instead of exiting cleanly. The latest apple/container release (v0.10.0) pins containerization 0.26.3 and does not include these fixes.

Separately, Wait was listening for OS signals directly alongside a ctx.Done() case that only waited for the process without stopping the container — the ctx.Done() path would block forever if the container hung. Consolidating to ctx.Done() with a container stop call fixes this.

How to Test

1. Run contagent against the apple runtime: contagent <command>
2. Press Ctrl-C while the container is running
3. Verify the container exits cleanly and does not hang

Notes

The login shell workaround is intentionally temporary. Removal steps are documented in a TODO comment in container.go.