[linear] Require opt-in labels for dispatch (#88)

#### Context

Projects need an explicit opt-in signal so active issues are not
dispatched only because they belong to a polled project.

#### TL;DR

*Allow workflows to require Linear labels before an issue is dispatched
or continued.*

#### Summary

- Add normalized `tracker.required_labels` configuration with all-label
matching
- Apply the label gate to dispatch, retries, reconciliation, and
continuation turns
- Document the configuration and cover opt-in and label-removal behavior

#### Alternatives

- Prompt-only filtering happens after an issue is claimed and cannot
stop active work when a label is removed.
- Linear query filtering would hide removed labels from reconciliation,
preventing clean agent shutdown.

#### Test Plan

- [x] `make -C elixir all`
- [x] `mix test test/symphony_elixir/workspace_and_config_test.exs`