Fix Slack upload default target comments

[decofe]

Summary

• avoid indexing the optional files argument when Slack upload defaults to the current thread
• keep upload comments attached only to the first normalized upload path
• propagate blocks-mode thread_key into CENTAUR_THREAD_KEY inside harness-server so shell commands like slack upload <file> can infer the Slack destination from warm or cold sandboxes
• add regression coverage for the Slack CLI single-file path and blocks-mode thread-key parsing

Prompted by: @Rjected

Tests

• cargo test --manifest-path crates/harness-server/Cargo.toml
• PYTHONPATH=$PWD/tools/productivity:$PWD uv run --with pytest --with typer --with slack-sdk --with python-dotenv --with rich --with structlog pytest tools/productivity/slack/tests/test_cli.py