refactor(session): replace MigrationEngine and isolate migrate command

[ousamabenyounes]

Summary

Replace the one-method MigrationEngine with previewMigration(session, sourcePlatform, targetPlatform) beside fidelityFromSession. The migration command calls the function directly; copy behavior still lives in AgentSession.copy.

Move migration command registration into its own module and shared session helpers into another, leaving session-cmd.ts focused on the remaining commands. The extracted command bodies are unchanged apart from imports, exports, and the preview call.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature causing existing behavior to change)
• Documentation only
• Refactor / internal cleanup

Test Plan

• npx tsc --noEmit passes
• npm run lint passes
• npx vitest run passes
• Added/updated tests for the change

A new contract test failed on the unmodified base (previewMigration is not a function: 1 failed, 6 passed). With the implementation, both focused suites passed (72 tests). Reverting the implementation reproduced the same failure. This RED verifies the requested refactor contract; it is not evidence of a pre-existing user-facing bug.

On the final commit, 72 focused tests and the real CLI migration E2E with OpenCode 2 passed. The changed executable logic has 17/17 covered lines; exact code moves were audited separately.

Full local run-ci.sh (baseline comparison): lint, typecheck, and build pass. The host suite has 6 failures on both base and PR; the only PR-only failure passed on isolated rerun and with its entire 166-test file. Every Node 20/22 failure in this run also occurred in at least one of the base branch’s Node 20/22 runs; Node 22 has 224 versus 225 on base. The full E2E suite has the same single failing test on base and PR (1 failed, 150 passed, 6 skipped test files on each).

Related Issues

Refs #1140, proposal 3 only. Proposals 1, 2, and 4 remain open.

Notes for Reviewers

The second commit moves the existing migration command and shared helpers out of session-cmd.ts; it does not rewrite their behavior. The API surface change is the removal of MigrationEngine in favor of previewMigration.

[github-actions]

No findings in the specified PR diff.

• The extracted migration command preserves its behavior apart from replacing MigrationEngine.preview with previewMigration.
• The PR description documents sufficient testing, including a representative real-CLI migration E2E with OpenCode 2.

Review was read-only; I did not run, build, or install PR code.

[ousamabenyounes]

Validation on c4e4f24: the new preview contract was RED on the unmodified base (1 failed, 6 passed), then GREEN (72 focused tests passed). The real CLI migration E2E passed with OpenCode 2. GitHub's six Node jobs, fork-safe E2E, build, metrics, and scan are green. The local full-suite baseline failures and the isolated transient test are documented in the PR description.