Add the SDK and CLI for the name-addressed app config fragment purge

## Objective

Make the name-addressed purge reachable from the client: two typed SDK v2 calls and the CLI commands over them, so removing a fragment by name is a single call rather than a read-then-purge composition.

## Background

The my app-config-fragment group reads and writes by config name but offers no delete, so removing one of my own fragments means resolving its id first and then leaving the my group for the top-level purge --id. With the server route in place the CLI can address the same (scope, config_name) pair that get and update already do.

## Acceptance Criteria

- bai my app-config-fragment purge CONFIG_NAMES... purges the caller's own user-scope fragments for those names.
- bai app-config-fragment purge-by-names --scope-type ... [--scope-id ...] CONFIG_NAMES... purges one named scope's fragments.
- A name with no fragment at the addressed scope fails the command and purges nothing.
- Both SDK calls land on the my and scoped by-names bulk-delete routes and parse the payload.
- Both commands are registered in the /bai-cli skill entity-command reference.

## Related

- BA-7149 - the originating issue, whose acceptance criteria this issue carries forward.
- src/ai/backend/client/cli/v2/AGENTS.md

## Story Points

0.5

JIRA Issue: BA-7185

Start with src/ai/backend/client/cli/v2/AGENTS.md and the existing my and scoped app-config-fragment get/update commands, then trace their SDK routes. Check the /bai-cli skill entity-command reference for registration requirements. Done means both typed SDK calls parse their bulk-delete responses, both CLI commands handle the named-scope behavior, and the no-fragment case purges nothing.

Written by the indexing model from the issue text.