feat: add migration benchmarking + terminal-forward README

[forkadarshp]

Add an optional, opt-in benchmarking capability that quantifies a migration with measured evidence instead of assertions.

Skill changes:

• Ask once in Phase 0 whether the user wants pre/post benchmark stats in the evidence report; capturing the baseline later is impossible, so it is gated up front.
• Three-arm methodology wired into the existing phase boundaries:
  • Arm A (baseline: old model + old prompts) captured in Phase 0, pre-edit
  • Arm B (naive swap: new model + old prompts) captured mid Phase 4, after runtime edits but before prompt tuning -- the honest raw-model control
  • Arm C (new model + enhanced system) captured in Phase 5
• New references/benchmarking.md: dimensions (quality, output-contract, tool calling, latency, cost, safety, robustness), composite scoring, honesty rules, and leaderboard format.
• Phase 6 report gains an optional Benchmark Results section; anti-patterns and the validation checklist updated; validate_skill.py now requires the new doc.
• examples/example-output.md shows a filled three-arm leaderboard (clearly labelled illustrative).

README:

• Replace the bland Problem/Solution prose with a terminal-style hero session contrasting the naive find/replace swap with the ModelPort migration.
• New "Benchmark your migration" section with an illustrative leaderboard and a link to the methodology; benchmark added to the feature table, why-teams list, expected-output list, repo structure, and roadmap.

All benchmark numbers in docs are illustrative and labelled as such; latency and cost are described as measured-not-assumed. validate_skill.py passes and markdownlint is clean across all 25 markdown files.

Pull Request

Summary

Describe what changed and why.

Type of change

• Bug fix
• New feature
• Docs update
• Refactor

Migration relevance

• API/runtime compatibility
• Prompt behavior
• Agent/tool behavior
• Tests/docs sync
• Open-source packaging or discoverability
• Breaking changes

Validation

• python3 scripts/validate_skill.py .
• Local checks completed
• Relevant examples/docs updated
• No unrelated changes included

Notes for reviewers

Anything reviewers should focus on.