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Prompt targets are no longer actively maintained.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.

cli/planoai/main.py

@@ -7,7 +7,6 @@ import contextlib
 import logging
 import rich_click as click
 import yaml
-from planoai import targets
 from planoai.defaults import (
     DEFAULT_LLM_LISTENER_PORT,
     detect_providers,
@@ -622,28 +621,6 @@ def down(docker, verbose):
                 )
 
 
-@click.command()
-@click.option(
-    "--f",
-    "--file",
-    type=click.Path(exists=True),
-    required=True,
-    help="Path to the Python file",
-)
-def generate_prompt_targets(file):
-    """Generats prompt_targets from python methods.
-    Note: This works for simple data types like ['int', 'float', 'bool', 'str', 'list', 'tuple', 'set', 'dict']:
-    If you have a complex pydantic data type, you will have to flatten those manually until we add support for it.
-    """
-
-    print(f"Processing file: {file}")
-    if not file.endswith(".py"):
-        print("Error: Input file must be a .py file")
-        sys.exit(1)
-
-    targets.generate_prompt_targets(file)
-
-
 @click.command()
 @click.option(
     "--debug",
@@ -741,7 +718,6 @@ main.add_command(down)
 main.add_command(build)
 main.add_command(logs)
 main.add_command(cli_agent)
-main.add_command(generate_prompt_targets)
 main.add_command(init_cmd, name="init")
 main.add_command(trace_cmd, name="trace")
 main.add_command(chatgpt_cmd, name="chatgpt")