feat(cli)!: make init the single entry point, fold in and remove create - #4883

Trim the orientation paragraph to what the human reader needs — this file
is yours; AGENTS.md/CLAUDE.md are the agent's guide, picked up
automatically. Drop the breakdown of what AGENTS.md teaches the agent.

`pipecat init` is now the single entry point for building a Pipecat app.
It still writes the coding-agent guide (AGENTS.md + CLAUDE.md) and now
also scaffolds a runnable bot — interactively, or non-interactively from
flags or a config file:

    pipecat init . --bot-type web -t daily --mode cascade \
      --stt deepgram_stt --llm openai_llm --tts cartesia_tts

Scaffolding is directory-first and in-place (project name derived from the
target dir); create's --output/-o and --name-subfolder layout are gone. A
missing target with scaffold flags defaults to the current directory so an
automated run never hangs on a prompt.

The scaffolder internals move to pipecat/cli/scaffold.py. `pipecat create`
is removed; a hidden stub points users to `pipecat init`.

… flags

Split run_non_interactive_scaffold into resolve_scaffold_config (merge +
validate + --dry-run) and generate_scaffold (write), so `pipecat init`
validates before writing the agent guide. An incomplete invocation like
`pipecat init . --bot-type web` now fails atomically instead of dropping
AGENTS.md/CLAUDE.md and then erroring.

Also warn when scaffold flags are passed to `pipecat init quickstart` (a
fixed preset) instead of silently ignoring them.

AGENTS.md is the coding-agent guide; the bare/quickstart interactive forms
are for humans (covered in GETTING_STARTED.md) and the scaffold section's
warning already tells the agent to avoid the interactive form.

Make the three guide files `pipecat init` writes (AGENTS.md, CLAUDE.md,
GETTING_STARTED.md) follow one uniform rule instead of asymmetric ones.
Previously AGENTS.md and GETTING_STARTED.md were always overwritten while
CLAUDE.md was preserved — which silently destroyed a non-Claude user's
hand-edited AGENTS.md on every re-run.

Now: absent -> write; present -> keep it (never clobber silently);
`--overwrite-guide` (renamed from `--force`, now covering all three files)
to overwrite. A re-run reports what it did per file, and if a kept guide
carries an older Pipecat's version stamp it prints a heads-up pointing at
`--overwrite-guide` — so staleness is surfaced, not silent.

- init.py: `_write_guide_file` helper (absent/keep/overwrite) + `_stamped_version`
  + `_print_refresh_summary`; footer text and flag/param renamed
- tests: re-run now preserves; add stale-nudge and summary coverage

Integrating preserve-by-default (this branch) with init's scaffold path
(base): scaffolding over an existing guide now keeps it instead of
refreshing it, so update the in-place test to assert preservation.

…te helpers

feat(cli): preserve existing guide files on `pipecat init` re-run