fix: isolate workspace tmp directories

[ayomidelog]

Summary

This PR fixes workspace /tmp isolation.

Before this change, workspaces had isolated /home mounts but still shared the same /tmp inherited from the sandbox rootfs. That meant one workspace could observe temporary files created by another workspace inside the same sandbox.

This PR gives each workspace its own private /tmp tmpfs during session bootstrap.

Problem

The current runtime model isolates:

• user namespace
• PID namespace
• mount namespace
• network namespace
• /home

But /tmp was still coming from the shared sandbox rootfs, so it behaved like shared sandbox state rather than per-workspace runtime state.

In practice, that meant:

• a file created in /tmp by workspace A could be visible in workspace B
• temporary data was not isolated to a single workspace
• /tmp semantics did not match expected workspace isolation

What Changed

Per-workspace /tmp tmpfs mount

During workspace bootstrap, Enclave now mounts a private tmpfs at /tmp for each workspace session.

The mount is configured with the expected temporary-directory semantics:

• private to the workspace runtime
• sticky-bit permissions (mode=1777)
• not shared through the sandbox rootfs

This change is separate from the existing runtime tmpfs mounts used for:

• /run/enclave/auth
• /run/enclave/env

User-Facing Impact

After this change:

• workspaces no longer share /tmp
• temporary files created in one workspace stay private to that workspace
• /tmp now behaves more like users expect from an isolated workspace environment

Tests Added

Added a root integration test:

• start two workspaces inside the same sandbox
• write a file into /tmp from workspace A
• verify workspace B cannot see that file

This is implemented as:

• integration::tmp_isolation::workspaces_do_not_share_tmp_directory_contents

Verification

Build and static checks

• cargo clippy --all-targets -- -D warnings

Default test suite

• cargo test --all-targets -- --skip integration:: --skip stress::

Root integration validation

• sudo env ... cargo test --test integration_suite -- --ignored --test-threads=1

The new tmp-isolation integration test passed. A later failure in the same ignored suite was due to an unrelated stale host-side veth collision during another network-heavy integration test, not due to the /tmp isolation change itself.

Why This Matters

/tmp is runtime state, not shared base-environment state.

If workspaces are meant to be isolated execution contexts, they should not be able to observe each other's temporary files just because they share a sandbox rootfs. This PR closes that gap and makes workspace filesystem behavior more coherent.