[TASK] Add index_names() or zero-arg indices() listing to public API

[mtauha]

Summary

indices(name) requires a name argument but there is no way to discover valid index names from the public API without reading source code or docs.

Problem

from psxdata import indices
indices()  # TypeError: missing 1 required positional argument: 'name'

A new user has no way to know what values are valid for name.

Proposed Fix (Phase 7)

Either:

• Add a zero-arg index_names() function that returns a list of valid index names (e.g. ['KSE100', 'KSE30', ...])
• Or make indices() with no args return the list of valid names with a deprecation hint

Document valid index names clearly in the API reference.

Context

Discovered during post-publish smoke test of v0.1.0a1.

Parent: Phase 7 (Documentation)