feat(core): Implement infrastructure layer for CLI operations

Establishes foundational modules that all other components depend on.

src/core/config.rs - Configuration management:
- JSON-based config file with Zod-like validation via serde
- GitLab settings: base URL, token environment variable
- Project list with paths to track
- Sync settings: backfill days, stale lock timeout, cursor rewind
- Storage settings: database path, payload compression toggle
- XDG-compliant config path resolution via dirs crate
- Loads GITLAB_TOKEN from configured environment variable

src/core/db.rs - Database connection and migrations:
- Opens or creates SQLite database with WAL mode for concurrency
- Embeds migration SQL as const strings (001-005)
- Runs migrations idempotently with checksum verification
- Provides thread-safe connection management

src/core/error.rs - Unified error handling:
- GiError enum with variants for all failure modes
- Config, Database, GitLab, Ingestion, Lock, IO, Parse errors
- thiserror derive for automatic Display/Error impls
- Result type alias for ergonomic error propagation

src/core/lock.rs - Distributed sync locking:
- File-based locks to prevent concurrent syncs
- Stale lock detection with configurable timeout
- Force override for recovery scenarios
- Lock file contains PID and timestamp for debugging

src/core/paths.rs - Path resolution:
- XDG Base Directory Specification compliance
- Config: ~/.config/gi/config.json
- Data: ~/.local/share/gi/gi.db
- Creates parent directories on first access

src/core/payloads.rs - Raw payload storage:
- Optional gzip compression for storage efficiency
- SHA-256 content addressing for deduplication
- Type-prefixed keys (issue:, discussion:, note:)
- Batch insert with UPSERT for idempotent ingestion

src/core/time.rs - Timestamp utilities:
- Relative time parsing (7d, 2w, 1m) for --since flag
- ISO 8601 date parsing for absolute dates
- Human-friendly relative time formatting

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/mod.rs

@@ -0,0 +1,12 @@
+//! Core infrastructure modules.
+
+pub mod config;
+pub mod db;
+pub mod error;
+pub mod lock;
+pub mod paths;
+pub mod payloads;
+pub mod time;
+
+pub use config::Config;
+pub use error::{GiError, Result};