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/deploy_key_provisioner.py

@@ -49,7 +49,7 @@ def get_provisioner(
             GiteaDeployKeyProvisioner,
         )
         return GiteaDeployKeyProvisioner(token=token, api_url=api_url)
-    from .manifest_util import ManifestError
+    from .manifest.util import ManifestError
     raise ManifestError(
         f"unknown provisioned_key provider: {provider!r}; "
         f"available: gitea"