test(cli): the sandbox holds when CLAUDE_CONFIG_DIR is set

[bdelanghe]

Tests pin $HOME to a tempdir, which sandboxes every reader that derives its paths from it. The Claude reader does not derive from $HOME: since b31f2c5 it prefers $CLAUDE_CONFIG_DIR, and the child process inherits that from whoever ran cargo.

On a machine that exports it — every Claude Code user, since that variable is how a custom config root gets selected at all — the CLI walks straight out of the sandbox:

Error: Conversation not found: /Users/bobby/.config/claude/projects/-tmp-.../session-abc.jsonl
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^ should have been the test's temp HOME

CI never sees this; it exports no such variable. Locally it is 28 failures:

cargo test -p path-cli --test integration               20 failed
cargo test -p path-cli --test query                      8 failed
env -u CLAUDE_CONFIG_DIR cargo test -p path-cli ...      all pass

The fix

Remove the variable in the three places each suite already builds its sandboxed command — integration.rs's cmd(), query.rs's cmd() and its two direct Command::cargo_bin call sites — and in ScopedHome, beside $HOME and $TOOLPATH_CONFIG_DIR.

Removed, not pinned: the point is to restore the reader's $HOME-relative default so the HOME the test already sets is the one that counts. ScopedHome restores the previous value on drop like the others. A test that wants to exercise the override sets it on its own command.

After this, one failure remains — deliberately

file_input_explicit_opencode_projects_and_records_exec still fails. It is the same shape of escape through a different variable: the opencode PathResolver prefers $XDG_DATA_HOME over $HOME, so it finds the developer's real opencode.db. The fix is a separate one-liner in ScopedHome — it was #142, whose branch died with the fork; the commit survives as tag archive/fork/fix-opencode-test-xdg-isolation and is a clean cherry-pick onto main.

Kept out of this PR: different variable, different reader, its own review.

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1. Independent PR — no bundled or speculative changes
   • Verified: one concern — $CLAUDE_CONFIG_DIR escaping the test sandbox. Test files only, no production code.
2. Changed codepaths verified — targeted unit and full integration tests
   • Verified: cargo test -p path-cli --test integration 62/62 and --test query 37/37, run with CLAUDE_CONFIG_DIR still exported — the condition that previously failed. cargo test --workspace goes from 29 failures to 1. cargo fmt --all --check clean.
3. Root cause identified — every failure traced to source
   • b31f2c5 taught the reader to prefer $CLAUDE_CONFIG_DIR; the test harness sandboxes $HOME only, so the variable was inherited and outranked the sandbox. Confirmed by env -u flipping every failure to a pass.
4. No duplication — refactoring preferred over copy/paste
   • Removed at the points that already assemble the sandbox rather than adding a new helper; ScopedHome follows the save/restore pattern already there for $HOME and $TOOLPATH_CONFIG_DIR.
5. No unrelated changes — housekeeping isolated to its own branch
   • The $XDG_DATA_HOME escape is left failing for its own PR

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