refactor(forge): address PR #318 review — PR/Issue split, sqlite state, drop footer

Addresses the five review comments on PR #318:

- Split PullRequest from Issue and add a dedicated read_pr method on
  Forge/ScopedForge/GiteaForge (a PR carries merge state an issue does
  not); is_pr_open now derives from read_pr.
- Replace the JSON-file forge state with a thin swappable CRUD interface
  (ForgeStateStore) backed by SQLite (SqliteForgeStateStore) at
  ~/.bot-bottle/bot-bottle.db.
- Remove the provenance footer (provenance.py + its test): a mutable,
  unsigned PR comment is not an audit record.
- Reword the PRD: provenance is exposed via an API, not surfaced in the
  PR; document the Issue/PullRequest split and the SQLite store.

pyright clean (whole repo), pylint 10/10, 38 forge/resume unit tests pass;
no remaining refs to the removed provenance module or old JSON state API.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WL77TgFxKbs3cidGMG9dz7

bot_bottle/contrib/forge/base.py

@@ -26,8 +26,7 @@ from dataclasses import dataclass
 
 @dataclass(frozen=True)
 class Issue:
-    """A forge issue or PR (forges model PRs as issues with the same
-    number)."""
+    """A forge issue (not a PR — see `PullRequest`)."""
 
     number: int
     title: str
@@ -35,6 +34,20 @@ class Issue:
     state: str  # "open" | "closed"
 
 
+@dataclass(frozen=True)
+class PullRequest:
+    """A forge pull request. Kept distinct from `Issue` even though some
+    forges model PRs as issues on the wire: the domain objects carry
+    different data (a PR has merge state) and are read through different
+    methods (`read_pr` vs `read_issue`)."""
+
+    number: int
+    title: str
+    body: str
+    state: str  # "open" | "closed"
+    merged: bool
+
+
 @dataclass(frozen=True)
 class Comment:
     id: int
@@ -53,7 +66,11 @@ class Forge(abc.ABC):
 
     @abc.abstractmethod
     def read_issue(self, number: int) -> Issue:
-        """Read an issue or PR body (read-anywhere)."""
+        """Read an issue body (read-anywhere)."""
+
+    @abc.abstractmethod
+    def read_pr(self, number: int) -> PullRequest:
+        """Read a pull request, including its merge state (read-anywhere)."""
 
     @abc.abstractmethod
     def read_comments(self, number: int) -> list[Comment]:
@@ -122,6 +139,9 @@ class ScopedForge(Forge):
     def read_issue(self, number: int) -> Issue:
         return self._inner.read_issue(number)
 
+    def read_pr(self, number: int) -> PullRequest:
+        return self._inner.read_pr(number)
+
     def read_comments(self, number: int) -> list[Comment]:
         return self._inner.read_comments(number)