Add Agent-Forge spec artifacts

[Ghost-Frame]

Summary

• extend Agent-Forge specs with optional unchanged behaviors, implementation tasks, and property-test candidates
• deterministically render requirements.md, design.md, and tasks.md from existing Forge records
• trace requirements through implementation tasks to passing verification evidence
• derive task checkboxes from evidence instead of storing mutable completion state
• expose matching spec_artifacts behavior through the CLI and local MCP

Compatibility and safety

• keep all new spec_task fields optional for existing clients
• migrate existing SQLite databases with backward-compatible default values
• render an explicit gap when an older bugfix spec has no unchanged-behavior entries
• never infer property-test candidates that were not explicitly supplied
• leak-screen all three generated documents before creating files
• use no Kiro code or runtime dependency

Verification

• 93 Agent-Forge tests passed with the fluency feature
• strict all-target Agent-Forge Clippy passed with warnings denied
• declaration comment checks and adversarial review completed for all 12 touched files
• repository-wide pre-push formatting, Clippy, and dependency-policy checks passed
• the clean post-merge branch is byte-for-byte equivalent to the previously verified implementation commit

Branch reconstruction

PR #217 was squash-merged. This branch was created from the resulting main commit and contains only the single spec-artifacts implementation commit.