chore: remove outdated JSON manifest and fix stale PRD references

- Remove bot-bottle.demo.json (unused artifact from pre-YAML-migration era)
- Update AGENTS.md to reflect current manifest system (YAML markdown in ~/.bot-bottle/)
- Fix stale docstring in test_docker_bottle.py that referenced superseded PRD 0021

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

tests/unit/test_docker_bottle.py

@@ -1,10 +1,9 @@
-"""Unit: DockerBottle's argv builder (PRD 0021 chunk 1).
+"""Unit: DockerBottle's argv builder.
 
-`agent_argv` is the pure helper that `exec_agent` and the
-PRD-0021 tmux helpers both build on. It encodes two non-trivial
-rules — the optional `--append-system-prompt-file` flag and the
-optional `-it` for TTY mode — that we lock down here so the tmux
-path can rely on identical behavior.
+`agent_argv` is the pure helper for constructing docker exec command
+arguments. It encodes two non-trivial rules — the optional
+`--append-system-prompt-file` flag and the optional `-it` for TTY mode
+— that we lock down here so callers can rely on consistent behavior.
 """
 
 from __future__ import annotations