feat: Phase 3 — stories data model, ValidStoryTransition, storage CRUD, API endpoints

- internal/task/story.go: Story struct, StoryState constants, ValidStoryTransition
- internal/task/task.go: add StoryID field
- internal/storage/db.go: stories table + story_id on tasks migrations; CreateStory,
  GetStory, ListStories, UpdateStoryStatus, ListTasksByStory; update all task
  SELECT/INSERT to include story_id; scanTask extended with sql.NullString for story_id;
  added modernc timestamp format to GetMaxUpdatedAt
- internal/storage/sqlite_cgo.go + sqlite_nocgo.go: build-tag based driver selection
  (mattn/go-sqlite3 with CGO, modernc.org/sqlite pure-Go fallback) so tests run
  without a C compiler
- internal/api/stories.go: GET/POST /api/stories, GET /api/stories/{id},
  GET/POST /api/stories/{id}/tasks (auto-wires depends_on chain),
  PUT /api/stories/{id}/status (validates transition)
- internal/api/server.go: register all story routes
- go.mod/go.sum: add modernc.org/sqlite pure-Go dependency

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/internal/storage/sqlite_nocgo.go b/internal/storage/sqlite_nocgo.go
new file mode 100644
index 0000000..9862440
--- /dev/null
+++ b/internal/storage/sqlite_nocgo.go
@@ -0,0 +1,21 @@
+//go:build !cgo
+
+package storage
+
+import (
+	"database/sql"
+	"database/sql/driver"
+
+	modernc "modernc.org/sqlite"
+)
+
+// Register the modernc pure-Go SQLite driver under the "sqlite3" name so that
+// the rest of the codebase can use sql.Open("sqlite3", ...) regardless of
+// whether CGO is enabled.
+func init() {
+	sql.Register("sqlite3", &modernc.Driver{})
+}
+
+// modernc.Driver satisfies driver.Driver; this blank-import ensures the
+// compiler sees the interface is satisfied.
+var _ driver.Driver = (*modernc.Driver)(nil)