feat: add light-tier plan alternative and architect model bands (deep/fast)

[IgorWnek]

Key points:

• Added a light-tier plan alternative: a canonical 7-condition tier predicate and a lightweight plan template, used when a change is low-risk (no schema/security/dependency changes, single small phase, etc.)
• Changed tsh-architect's default model to Claude Sonnet 5 (from Opus-primary)
• Introduced two generated architect variants — tsh-architect-deep (Opus) and tsh-architect-fast (Luna/Flash) — sharing the same body as tsh-architect, differing only in model band; a sync script keeps them in sync and CI enforces it
• Added explicit routing rules so tsh-engineering-manager knows when to delegate to each of the three architect bands (default, deep, fast)
• Added the repo's first CI workflow (PR-only) gating variant sync and the docs build
• Added root-level copilot-instructions.md documenting the repo structure and the variant-sync contract
• Gate code review on reviewable implementation artifacts; tsh-code-reviewer shouldn't be invoked on docs or plans.