Add GitHub Issues tracker adapter

#### Context

Symphony's generic tracker seam needs a real non-Linear stress test without adding generic mutation APIs.

#### TL;DR

*Add GitHub Issues reads and a host-authenticated raw GitHub tool.*

#### Summary

- Add project-scoped GitHub issue reads, normalization, and dispatchability.
- Add raw `github_api` for provider-native mutations with host-side auth.
- Add contract tests, live E2E proof, and GitHub adapter documentation.

#### Alternatives

- Generic CRUD was rejected because it would flatten GitHub-specific capabilities.
- GraphQL was rejected for v1 because REST covers the required reads and tool flow simply.

#### Test Plan

- [x] `make -C elixir all`
- [x] `SYMPHONY_RUN_GITHUB_LIVE_E2E=1 mix test test/symphony_elixir/github_live_e2e_test.exs`
- [x] `SYMPHONY_RUN_LIVE_E2E=1 SYMPHONY_LIVE_LINEAR_TEAM_KEY=FST mix test test/symphony_elixir/live_e2e_test.exs:123`