Add v2 signal-based runtime and migration documentation

[drexed]

🎟️ Issue, exception, or other link

N/A

📓 Change description

This branch delivers CMDx 2.0.0: a full runtime rewrite documented in CHANGELOG.md and docs/v2-migration.md.

• Runtime & API: Signal-based execution (catch/throw), CMDx::Runtime, frozen Result backed by Signal, fiber-local Chain, Task#work as the subclass hook with Task.execute / execute! entry points, telemetry events, rollback and saga-style pipeline compensation, parallel groups with :executor / :merger, :continue_on_failure, retriers/deprecators registries, around_execution / after_execution callbacks, and related configuration (strict_context, log_exclusions, correlation xid, etc.).
• Inputs/outputs: Replaces the v1 attribute stack with Input/Inputs and adds Output/Outputs; docs and skills references moved from docs/attributes/* to docs/inputs/* and new outcomes/interruptions docs.
• Quality-of-life: Fault matchers and reason i18n path, Util for conditions, JSON helpers on core value objects, pattern-matching deconstruct support, and broad spec/integration rewrites to match the new surface.
• Tooling & packaging: Ruby version bump, CI/docs workflow tweaks, RuboCop updates, benchmark harness under benchmark/, trimmed bundled locales, README and copilot instructions refresh, and Cursor agent/skill/command assets under .cursor/.

🧪 Test plan

• bundle install (Ruby version per .ruby-version)
• bundle exec rubocop .
• bundle exec rspec .
• Optional: run benchmark scripts under benchmark/ if validating performance-related changes
• Skim docs/v2-migration.md and generated docs site (MkDocs) for broken links or obvious typos

✅ Before you request a review

• Tests added or updated (spec/ and spec/integration/ overhauled for v2)
• Documentation updated (docs/, README.md, CHANGELOG.md, skills/)
• CHANGELOG.md updated for user-visible changes
• CI green on this PR (GitHub Actions)
• Breaking changes called out for reviewers (see CHANGELOG.md BREAKING entries and docs/v2-migration.md)

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