fix(config): seed --catalog-name from default-catalog in config file

[rasta-rocket]

Problem

The CLI --catalog-name flag defaults to "default". When a config file used a different name under default-catalog, that value was never read ‚ ParseConfig was called with the flag's default before the config file had any chance to influence it. The result was a silent fallback to the rest catalog type with no URI, producing a confusing error:

Get "v1/config": unsupported protocol scheme ""

Example config that triggered the bug:

default-catalog: my-catalog
catalog:
  my-catalog:
    type: glue
    warehouse: s3://my-bucket

Running iceberg list would fail because my-catalog was never found in the catalog map, mergeConf was never called, and the catalog type stayed as rest.

Fix

• Add config.ParseDefaultCatalog to extract default-catalog from the YAML without requiring a catalog name upfront.
• In main, load the config bytes once, then seed args.CatalogName from default-catalog when --catalog-name was not explicitly passed on the command line. The explicit-flag guard ensures CLI flags still take precedence.

How to test

1. Create ~/.iceberg-go.yaml with a non-"default" catalog name:

default-catalog: my-glue
catalog:
  my-glue:
    type: glue
    warehouse: s3://my-bucket

2. Run without --catalog-name:

AWS_PROFILE=my-profile AWS_REGION=my-region iceberg list

2. Before: Get "v1/config": unsupported protocol scheme ""
   After: connects to the Glue catalog correctly.

3. Verify explicit flag still wins:

iceberg --catalog-name other-catalog list

4. Should use other-catalog, not my-glue.

[zeroshade]

LGTM. Precedence is right: explicit --catalog-name wins, then the config file's default-catalog, then the built-in default; the nil-config and empty-default-catalog cases resolve correctly too. A couple of small nits inline, none blocking.

[zeroshade]

nit: this unmarshals the same bytes that ParseConfig re-unmarshals on the very next line - two parses of the same config per run. Harmless for a CLI, but you could return both the *CatalogConfig and the default-catalog name from a single parse and drop this helper. Also, since it's newly exported, a one-line doc comment would be good.

[zeroshade]

nit: this seeding path (seed from default-catalog when the flag is absent, explicit flag wins) is exactly what the PR fixes, but it isn't covered by a test. A small args_test.go case asserting args.CatalogName is seeded when --catalog-name is omitted (and left alone when it's explicit) would guard against regressions.