validate retrieve --k must be > 0

[lesbass]

Summary

Use an argparse custom type to reject zero and negative values for retrieve --k, producing a clear argparse-style error.

Closes #54.

Changes

• src/contextseek/cli/main.py: Added _positive_int() custom type function and applied it to retrieve --k
• tests/unit_tests/test_cli.py: Added tests for _positive_int and for the retrieve parser rejecting --k 0 and --k -1

Verification

Minimal standalone test confirms the behavior:

$ contextseek retrieve --k 0
error: argument --k: invalid positive int value: '0' (must be > 0)

$ contextseek retrieve --k -1
error: argument --k: invalid positive int value: '-1' (must be > 0)

$ contextseek retrieve --k 5
(works as before)

[Teingi]

LGTM