v2.7.0 — github_weekly_digest flagship tool

New flagship composite tool: github_weekly_digest — one call answers 'what happened this week in owner/repo' (releases + merged PRs + new issues + commits, configurable look-back window).

Real-API finding: repositories with pull requests disabled (deepseek-ai/deepseek-harness itself!) made the digest 404 entirely. Fixed with per-surface 404 tolerance + regression test.

Verification: 43 unit tests; live digest run against the real GitHub API returned 30 real commits; CI installs the bundle into a fresh dsh profile and boots dsh web.