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.

This follows the Snap data-location pattern for app-owned per-user data: snapd exposes $SNAP_USER_COMMON as user data shared across revisions, and snapcraft.yaml supports app-level runtime environment variables for directing commands to those locations. See:

• https://snapcraft.io/docs/reference/administration/data-locations/
• https://snapcraft.io/docs/reference/development/environment-variables/
• https://documentation.ubuntu.com/snapcraft/9.0/reference/project-file/snapcraft-yaml/#apps-app-name-environment

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.
• Update Snap installation docs to remove stale --classic examples and document the snap-owned CLI storage location.

Testing

• uv run --frozen pytest python/openshell/release_formula_test.py -q
• mise run pre-commit passes
• mise exec -- markdownlint-cli2 docs/about/installation.mdx --no-globs
• 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.

[krishicks]

In #1995 I had some docs updates that might be worth incorporating into this change; let me know!

[zyga]

This looks good. We may want to look at the gateway as well, but it doesn't have to be done here as it would affect migration.

[github-actions]

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