Split READMEs per package and resolve flowvault README review (#396)

* Split monolithic README into per-package docs for skyvault and flowvault

Ports the README restructure from devesh/v2-flowvault-readmes onto this
release branch, which already carries the v2 -> skyvault rename:

- root README.md becomes a short landing page pointing at each package
- skyvault/README.md holds the full skyflow-java documentation
- flowvault/README.md documents the skyflow-flowvault-java bulk surface

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Address README review: self-contained flowvault docs, per-record error model

Resolves review comments on the flowvault README:

- Quickstart declared `VaultController vault` twice. The second line also
  called `skyflowClient.vault("<VAULT_ID>")`, which flowvault does not
  expose — its `vault()` takes no argument and always resolves to the
  first configured vault. Both fixed, and the single-vault behaviour is
  now stated.
- Auth, Credentials, VaultConfig and error handling lived only in the
  skyvault README, so flowvault users had to switch documents to get
  started. flowvault now documents them inline: credential types and
  their setters, credential resolution order, bearer token generation,
  a full VaultConfig / Skyflow.builder() reference, timeout and retry
  resolution with defaults, and logging. Only the exotic token variants
  (context, scoped, signed) still point at skyvault.
- Added a note that flowvault is a new SDK versioned independently, so a
  1.x flowvault next to a 2.x skyvault does not read as a downgrade. In
  both the flowvault README and the root package table.
- Rewrote Error Handling around flowvault's actual model: request-level
  SkyflowException versus per-record success/failure inside a call that
  returns normally, including the trap that a call where every record
  failed still does not throw. Documents index/httpCode/error/requestId
  per record, the per-operation summaries, consumption loops for both
  the one-level and two-level (tokenize) shapes, and the retry helpers.

Also brings the API surface in line with this branch, which renamed
several types after the README was first written: BulkInsertRequestRecord
(was BulkInsertRecord), tableName (was table), UpsertOptions (was
upsert/upsertType), TokenGroupRedactions (was BulkTokenGroupRedactions),
Bulk*Options (was InsertOptions/TokenizeOptions/...), CustomHeaderKey
.SkyflowAccountId/.RequestIdHeader, and the response shape, which is now
a single `records` list rather than separate `success` and `errors` lists.

Verified: all 20 Java snippets compile against the branch's classes, and
every sample JSON response matches the real toString() output.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Document bulk batch size and concurrency configuration

The flowvault README noted that bulk calls are split into batches sent
concurrently, but never documented how to configure that. Add a
"Batching and concurrency" section covering the per-operation env vars,
their defaults and maximums, and how effective values are resolved.

Values verified against Constants.java and the configure*ConcurrencyAndBatchSize
methods in VaultController: batch size is min(value, max) with a warning-and-
clamp above the max and a warning-and-default for invalid input; concurrency is
additionally capped at ceil(itemCount / batchSize), so it never exceeds the
number of batches to run.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Devesh-Skyflow