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Prompt targets are no longer actively maintained.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/resources/cli_reference.rst

@@ -16,7 +16,6 @@ Quick Navigation
 - :ref:`cli_reference_logs`
 - :ref:`cli_reference_init`
 - :ref:`cli_reference_trace`
-- :ref:`cli_reference_prompt_targets`
 - :ref:`cli_reference_cli_agent`
 
 
@@ -260,24 +259,6 @@ Inspect request traces from the local OTLP listener.
 - ``--list`` cannot be combined with a specific trace-id target.
 
 
-.. _cli_reference_prompt_targets:
-
-planoai prompt_targets
-----------------------
-
-Generate prompt-target metadata from Python methods.
-
-**Synopsis**
-
-.. code-block:: console
-
-   $ planoai prompt_targets --file <python-file>
-
-**Options**
-
-- ``--file, --f <python-file>``: required path to a ``.py`` source file.
-
-
 .. _cli_reference_cli_agent:
 
 planoai cli_agent