Add comments feature (bd-162)

- Add comments table to SQLite schema
- Add Comment type to internal/types
- Implement AddIssueComment and GetIssueComments in storage layer
- Update JSONL export/import to include comments
- Add comments to 'bd show' output
- Create 'bd comments' CLI command structure
- Fix UpdateIssueID to update comments table and defer FK checks
- Add GetIssueComments/AddIssueComment to Storage interface

Note: CLI command needs daemon RPC support (tracked in bd-163)
Amp-Thread-ID: https://ampcode.com/threads/T-ece10dd1-cf64-48ff-9adb-dd304d0bcb25
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/schema.go

@@ -56,6 +56,19 @@ CREATE TABLE IF NOT EXISTS labels (
 
 CREATE INDEX IF NOT EXISTS idx_labels_label ON labels(label);
 
+-- Comments table
+CREATE TABLE IF NOT EXISTS comments (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    issue_id TEXT NOT NULL,
+    author TEXT NOT NULL,
+    text TEXT NOT NULL,
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
+);
+
+CREATE INDEX IF NOT EXISTS idx_comments_issue ON comments(issue_id);
+CREATE INDEX IF NOT EXISTS idx_comments_created_at ON comments(created_at);
+
 -- Events table (audit trail)
 CREATE TABLE IF NOT EXISTS events (
     id INTEGER PRIMARY KEY AUTOINCREMENT,