feat: add SQLite cache store for incremental session persistence

Implement the caching layer that enables fast subsequent runs by
persisting parsed session data in SQLite:

- store/schema.go: DDL for three tables — sessions (primary metrics
  with file_mtime_ns/file_size for change detection), session_models
  (per-model breakdown, FK cascade on delete), and file_tracker
  (path -> mtime+size mapping for cache invalidation). Indexes on
  start_time and project for efficient time-range and filter queries.

- store/cache.go: Cache struct wrapping database/sql with WAL mode
  and synchronous=normal for concurrent read safety and write
  performance. Key operations:

  * Open: creates the cache directory, opens/creates the database,
    and ensures the schema is applied (idempotent via IF NOT EXISTS).

  * GetTrackedFiles: returns the mtime/size map used by the pipeline
    to determine which files need reparsing.

  * SaveSession: transactional upsert of session stats + model
    breakdown + file tracker entry. Uses INSERT OR REPLACE to handle
    both new files and files that changed since last parse.

  * LoadAllSessions: batch-loads all cached sessions with a two-pass
    strategy — first loads session rows, then batch-loads model data
    with an index map for O(1) join, avoiding N+1 queries.

  Uses modernc.org/sqlite (pure-Go, no CGO) for zero-dependency
  cross-platform builds.

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

internal/store/schema.go

@@ -0,0 +1,49 @@
+package store
+
+const schemaSQL = `
+CREATE TABLE IF NOT EXISTS sessions (
+    session_id           TEXT PRIMARY KEY,
+    project              TEXT NOT NULL,
+    project_path         TEXT,
+    file_path            TEXT NOT NULL,
+    is_subagent          INTEGER NOT NULL DEFAULT 0,
+    parent_session       TEXT,
+    start_time           TEXT,
+    end_time             TEXT,
+    duration_secs        INTEGER,
+    user_messages        INTEGER,
+    api_calls            INTEGER,
+    input_tokens         INTEGER,
+    output_tokens        INTEGER,
+    cache_creation_5m    INTEGER,
+    cache_creation_1h    INTEGER,
+    cache_read_tokens    INTEGER,
+    estimated_cost       REAL,
+    cache_hit_rate       REAL,
+    file_mtime_ns        INTEGER NOT NULL,
+    file_size            INTEGER NOT NULL,
+    parsed_at            TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS session_models (
+    session_id           TEXT NOT NULL REFERENCES sessions(session_id) ON DELETE CASCADE,
+    model                TEXT NOT NULL,
+    api_calls            INTEGER,
+    input_tokens         INTEGER,
+    output_tokens        INTEGER,
+    cache_creation_5m    INTEGER,
+    cache_creation_1h    INTEGER,
+    cache_read_tokens    INTEGER,
+    estimated_cost       REAL,
+    PRIMARY KEY (session_id, model)
+);
+
+CREATE TABLE IF NOT EXISTS file_tracker (
+    file_path            TEXT PRIMARY KEY,
+    mtime_ns             INTEGER NOT NULL,
+    size_bytes           INTEGER NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_sessions_start ON sessions(start_time);
+CREATE INDEX IF NOT EXISTS idx_sessions_project ON sessions(project);
+`