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/manifest/util.py

@@ -0,0 +1,24 @@
+"""Shared manifest primitives used by all manifest sub-modules."""
+
+from __future__ import annotations
+
+from typing import cast
+
+
+class ManifestError(Exception):
+    """A manifest file (or the manifest tree) is invalid."""
+
+
+def as_json_object(value: object, label: str) -> dict[str, object]:
+    """Assert that `value` is a JSON object (str-keyed dict) and return
+    a view typed as `dict[str, object]` so downstream `.get(...)` calls
+    have a typed surface."""
+    if not isinstance(value, dict):
+        raise ManifestError(f"{label} must be a JSON object (was {type(value).__name__})")
+    items = cast(dict[object, object], value)
+    out: dict[str, object] = {}
+    for k, v in items.items():
+        if not isinstance(k, str):
+            raise ManifestError(f"{label} keys must be strings (found {type(k).__name__})")
+        out[k] = v
+    return out