Implement external tool evaluation profiles

[fall-out-bug]

Summary

• Adds dated external tool evaluation profiles for CodeGraph, Understand-Anything, and ast-index.
• Adds bounded context-pack guidance so profiles stay adoption guidance, not evidence.
• Adds SpecKit plan/tasks/review dispositions for spec 084.

Verification

• go test -count=1 ./internal/contextprep
• go test ./...
• go vet ./...
• jq empty schema/*.json
• git diff --check
• local context prepare smoke with /tmp/portolan-084-context

Review state

• Planning review: opencode Kimi, GLM, MiniMax lanes recorded.
• Story reviews: US1/US2/US3 dispositions recorded.
• PR review: pending.

Merge readiness

• merge_approval: not_assessed
• GitHub checks: pending/not_assessed until PR checks run