Major upgrade to v2 (#305)

Major version bump driven by a Fern Python generator upgrade (`5.12.1`) that
introduced non-additive Python type changes and a behavioral retry default
change that aren't safe under semver as a patch or minor.

The public namespaced API (`client.actions.run(...)`, `client.proxy.get(...)`,
`client.workflows.invoke(...)`, etc.) and the `Pipedream` / `AsyncPipedream`
wrappers are unchanged; most v1.x application code will run on v2.x without
modification.

Highlights:

* Python 3.10+ is now required (was 3.8+).
* `ConfigurableProp_*` and `Emitter_*` discriminator aliases are removed. Use
  the un-prefixed variants.
* `ConfigurablePropDiscord` (root variant) and
  `pipedream.core.{Sync,Async}CustomPager` are removed.
* Default retry set expands to include `409 Conflict`. Pass `max_retries=0` on
  mutating calls where duplicate side effects matter.
* New: `pip install pipedream[aiohttp]` extra, structured logging via
  `logging=`, and first-class `headers` / `max_retries` / `follow_redirects` /
  `httpx_client` kwargs on the wrappers.
* `MIGRATE.md` guide for users of the v1.x SDK
* Add `CLAUDE.md` guide

---------

Co-authored-by: fern-api[bot] <115122769+fern-api[bot]@users.noreply.github.com>
Co-authored-by: Jay Vercellone <jay.vercellone@workday.com>