fix(snap): use snap-owned XDG directories

[pimlock]

Summary

Fix the Ubuntu Snap release canary failure where the snapped openshell CLI tried to write gateway metadata under the host ~/.config/openshell and hit snap confinement. The snap now points user-facing OpenShell commands at persistent snap-owned XDG directories under $SNAP_USER_COMMON.

Related Issue

No existing issue found. Investigated failing Release Canary run: https://github.com/NVIDIA/OpenShell/actions/runs/27986564035

Changes

• Set XDG_CONFIG_HOME, XDG_DATA_HOME, and XDG_STATE_HOME for the openshell and term snap apps.
• Add a package regression assertion that both user-facing snap apps carry the snap-owned XDG directory configuration.

Testing

• uv run --frozen pytest python/openshell/release_formula_test.py -q
• mise run pre-commit passes (skipped by request; the local workspace has unrelated untracked files that caused markdown/license checks to fail, and unrelated Rust check failures were also present)
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable)

[pimlock]

@zyga @drew This canary has been failing for a while now with the same error: https://github.com/NVIDIA/OpenShell/actions/workflows/release-canary.yml

I'm not sure if fixing this was waiting on something, but couldn't find anything in flight, so got the most straightforward fix in here. Happy to close if there is anything else coming.