refactor: port status page migrate to SDK + RunContext

[debidong]

Summary

Retires the CLI-local raw-HTTP transport introduced in #1 and routes the four status page migrate subcommands through flashduty-sdk@v0.7.0 and the existing RunContext / runCommand pattern. After this PR, 100% of the CLI flows through ctx.Client — no outlier transport.

Changes

SDK bump

• github.com/flashcatcloud/flashduty-sdk v0.6.0 → v0.7.0 (adds status page migration methods)

Interface extension (internal/cli/root.go)
Four methods added to flashdutyClient:

• StartStatusPageMigration
• StartStatusPageEmailSubscriberMigration
• GetStatusPageMigrationStatus
• CancelStatusPageMigration

Command rewrite (internal/cli/status_page_migrate.go, −455 / +150)

• Deleted: statusPageMigrationAPI, statusPageMigrationService, newStatusPageMigrationService, migrationEnvelope[T], local HTTP transport, manual sanitization helpers (sanitizeMigrationBody, redactAppKey, etc. — redundant now that SDK handles this), duplicate local types (migrationStartResult, migrationProgress, migrationJob).
• Each RunE uses runCommand(cmd, args, func(ctx *RunContext) error {...}).
• printMigrationStart / printMigrationStatus rewired from reading flagJSON / newPrinter() globals to ctx.JSON / ctx.Printer / ctx.Writer.

Test rewrite (internal/cli/status_page_migrate_test.go, −547 / +350)

• Replaced httptest.NewServer transport tests with newClientFn mock-swap + execCommand tests matching command_test.go convention.
• Wire-format coverage lives in flashduty-sdk's own tests — not duplicated here.
• Added mock stubs for the 4 new interface methods in command_test.go.

Post-review fixes (two independent reviewers)

go-reviewer + Codex both reviewed the uncommitted diff. All flagged issues resolved:

• Nil-guard mock methods so tests that override a subset of the migration methods fall back to mockClient instead of panicking.
• Strengthened error assertion for unsupported source: now also asserts "atlassian" appears in the supported list — catches silent drift.
• JSON contract preservation for cancel --json: emit both "command" (shipped by feat: add statuspage migration commands #1) and "next_command" (new canonical field) during a deprecation window.
• Validation ordering fix: validateMigrationSource runs before runCommand, so an invalid --from fails with the source error before any client/auth work — matches feat: add statuspage migration commands #1 behavior. Added TestCommandStatusPageMigrate{Structure,EmailSubscribers}ValidatesBeforeClient to lock the ordering.

Compatibility

Command output and JSON schemas remain backwards-compatible with what #1 shipped:

• Human output: identical text.
• JSON output for structure / email-subscribers: unchanged.
• JSON output for status: field names unchanged (maps directly to SDK's StatusPageMigrationJob).
• JSON output for cancel: "command" field retained + new "next_command" field added (pure addition).

Test plan

• go test -race -count=1 ./... green
• go test -tags=e2e -count=1 ./e2e/... green (24.4s)
• gofmt -l ., goimports -l ., go vet ./... clean
• Coverage on changed commands: 80–100% (printMigrationStatus at 57% due to defensive write-error returns that can't be triggered against bytes.Buffer)
• Two independent code reviews (go-reviewer agent + Codex)

Net

• -660 lines of raw HTTP / service / duplicate types / manual sanitization
• +400 lines of SDK-backed commands + focused mock-based tests
• CLI is now architecturally uniform.