refactor(manifest): consolidate the manifest modules into a bot_bottle.manifest package

Move the nine root-level manifest modules into a bot_bottle/manifest/ package,
dropping the redundant manifest_ prefix: the facade (manifest.py) becomes the
package __init__ and keeps re-exporting Manifest / ManifestIndex and the piece
types, while manifest_<x>.py become manifest/<x>.py (agent, bottle, egress,
extends, git, loader, schema, util).

Because the facade stays the package __init__, the ~13 callers that import
`from bot_bottle.manifest import ...` are unchanged; only direct-piece imports
move to `bot_bottle.manifest.<name>`. Inside the package, intra-manifest
imports drop the prefix and the non-manifest siblings (log, yaml_subset,
agent_provider) move to `..`.

No behavior change; full unit suite green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/cli/start.py

@@ -380,7 +380,7 @@ def _peek_agent_bottle(manifest: ManifestIndex, agent_name: str) -> str:
             return manifest.agents[agent_name].bottle
         return ""
 
-    from ..manifest_loader import scan_agent_names
+    from ..manifest.loader import scan_agent_names
     from ..yaml_subset import YamlSubsetError, parse_frontmatter
 
     home_agents = scan_agent_names(manifest.home_md / "agents")