docs+cli: deprecate prompt targets and remove generate_prompt_targets command (#944)

Prompt targets are no longer actively maintained. Mark them as deprecated in
the docs and remove the `planoai generate_prompt_targets` CLI command that
existed only to scaffold them.

Docs
- Add `.. deprecated::` banner to the Prompt Target concept page and to the
  function-calling guide / quickstart sections that walk users through
  configuring prompt targets.
- Relabel the Prompt Target card on the overview page as deprecated.
- Drop the Prompt Targets bullet from the README's Getting Started list.

CLI
- Remove the `generate_prompt_targets` Click command, its registration, and
  the `Utilities` rich-click command group.
- Delete `cli/planoai/targets.py` (the command's only consumer).
- Drop the `planoai prompt_targets` section from the CLI reference page.

Skills
- Delete the `cli-generate` rule, drop it from `plano-cli-operations`
  (description, when-to-use, rules list, execution checklist), and update
  the skills README. Hand-edit AGENTS.md to remove section 6.2 and renumber
  6.3/6.4 so the commit stays scoped (regenerating pulled in unrelated
  drift between rules/ and AGENTS.md).

The runtime gateway, schema, and existing demo configs still accept
`prompt_targets` blocks; this is deprecation, not removal of behavior.

docs/source/get_started/overview.rst

@@ -57,10 +57,10 @@ Deep dive into essential ideas and mechanisms behind Plano:
 
         Explore Plano's LLM integration options
 
-    .. grid-item-card:: :octicon:`workflow` Prompt Target
+    .. grid-item-card:: :octicon:`workflow` Prompt Target (Deprecated)
         :link: ../concepts/prompt_target.html
 
-        Understand how Plano handles prompts
+        Deprecated — kept for existing users. New apps should use Agents.
 
 
 Guides