docs(sdd-process,copilot): drop stale phase-planning language

Phases 3 and 4 are complete; their artifacts exist in the repo. The
narrative docs still spoke about them in the future tense.

- .guides/sdd-process.md: four occurrences of "Prompt (planned,
  Phase 4)" replaced with operational "Prompt." labels (lines 43,
  60, 77, 111). Stage 4 inputs reference dropped the parenthetical
  "(Phase 4)" suffix for prompt.test-cases.from-spec.yaml (line 90).
- .github/copilot-instructions.md:
  - Line 32: schema directory description now enumerates v2 as
    canonical, v1 as deprecated, plus the five SDD artifact schemas
    delivered in Phase 3 (and the test-cases schema landing in this
    polish pass).
  - Line 42: "v2 lands in Phase 3 of the roadmap" replaced with v2
    is canonical since Phase 3; v1 retained for backward
    compatibility during the v0.x window and not the target of new
    prompts.

No structural changes; reader-facing accuracy only.

Part of Phase 6 (v0.5.1 polish) — surfaced by the post-v0.5.0 audit.