Add MIGRATING.md and protect from regen overwrite

[highb]

Summary

• New `MIGRATING.md` at repo root: hand-written v1 → v2 upgrade guide
• `.genignore` entries for `MIGRATING.md` and `CHANGELOG.md` so Speakeasy never overwrites hand-written guides on regen

Why

Cutting v2.0.0-alpha.1 (see #109) introduces breaking changes that customers need clear guidance on:

1. Module path break (the biggest one): `import "github.com/conductorone/conductorone-sdk-go"` → `.../v2`. Required by Go's semantic import versioning for any v2.0.0+ release.
2. IGA-1181 schema renames: `AccessConflictNotificationConfig`, `AutomationsTaskAction`, `PolicyForm`, `AppOwnersV2` / `AppEntitlementOwnersV2` service splits.
3. New API surfaces added since v1.27.x: paper-secret, role-mining, principal/credentials, workload-federation, A2UI surfaces, automations expansion.

Layout

`MIGRATING.md` follows the convention used by aws-sdk-go-v2, gorm, and similar Go SDKs that have crossed major versions. Sections:

1. Module path break — concrete `go get` + sed-replace recipe
2. Renamed types — service splits and schema renames (some placeholders to fill in once Migrate to /v2 module path; pre-stage 2.0.0-alpha.1 bump #109's regen PR posts the full breaking-change list)
3. New services and fields (additive, opt-in)
4. Workflow / CI guidance for Dependabot/Renovate users
5. Issue reporting

Follow-ups

• Once PR #109's regen PR opens, populate the field-level breaking changes section with the actual `Breaking ⚠️` list from that PR's body
• Tag `v2.0.0-alpha.1` after regen merges (existing `speakeasy_sdk_publish.yml` workflow auto-tags on RELEASES.md push)

🤖 Generated with Claude Code

[linear]

IGA-1181 OpenAPI spec x-speakeasy-name-override collisions block SDK, TF, and doc generation

[highb]

irrelevant, these migrations no longer need to happen