docs: 8.0 migration guide and openfeature-provider pin

[eli-r-ph]

💡 Motivation and Context

The capture v1 stack (#1017-#1029) has breaking changes spread across eleven PRs. The major changeset already points users to "the migration guide", which does not exist yet. The OpenFeature provider also pins posthog<8.0.0, so it would block the upgrade.

Changes

• New docs/migration-7.x-to-8.0.md. It starts with a checklist of what needs code changes, then covers:
  • endpoints and self-hosting
  • removed options and APIs
  • errors and on_error
  • event uuids
  • event options and the layer order
  • AI capture
  • batching and size limits
  • the OpenFeature provider
• README: an 8.0 row in the version table and a link to the guide.
• The major changeset links to the guide.
• openfeature-provider-posthog accepts posthog>=7.0.0,<9.0.0, with its own changeset. The provider uses only flag APIs, which 8.0 does not change, so it does not need to force the upgrade.
• AGENTS.md: the capture summary now describes options as sent as given and layered. It used to say "strictly typed", which changed in feat!: per-event capture options and legacy property hoisting #1023.

No examples change: none use a removed option or API.

💚 How did you test it?

• Every public name in the guide was checked against the stack: CaptureError fields, module-level set_context_option and super_options, the 7.x positional constructor order, _TERMINAL_STATUSES including 429, and the backoff constants.
• The provider's CI job passes locally: uv sync, pytest, ruff, mypy, and the wheel smoke install on Python 3.10. The smoke install resolves posthog 7.x from PyPI, which an 8.x-only pin could not do before 8.0 is published.
• The root suite passes; no library code changed.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Written with Cursor (Claude Opus) under the direction of the assignee. Skills used: writing-user-facing-copy, writing-pr-descriptions.

Decisions for the reviewer:

• Provider bump is minor (0.1.x to 0.2.0) to mark posthog 8 support. patch is the alternative: Sampo already patch-bumps the provider on every posthog release.
• Pin is >=7.0.0,<9.0.0, not 8-only. Provider users can upgrade posthog when they choose, and the provider release does not depend on 8.0 being on PyPI first.
• The endpoints section tells self-hosted users to check that both v1 endpoints are served. It names no deployment type.
• The guide lives under docs/, a new directory. The repo had no earlier migration guide to follow.