fix(init): isolate integration-test XDG paths

[peterkc]

Summary

Init tests could inherit real config paths from the developer's machine. A test
could then write temporary repositories into the user's Gortex config.

This change sends all test config, state, runtime, and daemon files to temporary
directories. A regression test proves the inherited paths remain untouched.

Before: init test -> inherited user paths
After:  init test -> temporary test paths

Changes

• Add one helper that isolates all init test paths.
• Use it in every runInit integration test.
• Verify repeated init stays isolated and does not modify inherited paths.

Testing

• All tests pass (go test -race ./...) - the full race suite was not run.
• go test -race ./cmd/gortex -run '^TestInit' -count=1 - 22 tests passed.
• go test ./cmd/gortex -run '^TestInitCreatesProjectMarker$' -count=1 - passed.
• go test ./... - 12,077 tests passed across 158 packages.
• New tests added for new functionality.
• Benchmarks not run because the change is test isolation only.
• The production config SHA-256 was unchanged after the qualified full suite.

Checklist

• Code follows existing patterns in the codebase.
• No unnecessary abstractions added.
• Language extractor metadata is not applicable.
• Member-edge behavior is not applicable.

[zzet]

@peterkc Is any other work is expected or it accedentally stays as draft?